btrfs: fix some -Wmaybe-uninitialized warnings in ioctl.c

Jens reported the following warnings from -Wmaybe-uninitialized recent
Linus' branch.

  In file included from ./include/asm-generic/rwonce.h:26,
		   from ./arch/arm64/include/asm/rwonce.h:71,
		   from ./include/linux/compiler.h:246,
		   from ./include/linux/export.h:5,
		   from ./include/linux/linkage.h:7,
		   from ./include/linux/kernel.h:17,
		   from fs/btrfs/ioctl.c:6:
  In function ‘instrument_copy_from_user_before’,
      inlined from ‘_copy_from_user’ at ./include/linux/uaccess.h:148:3,
      inlined from ‘copy_from_user’ at ./include/linux/uaccess.h:183:7,
      inlined from ‘btrfs_ioctl_space_info’ at fs/btrfs/ioctl.c:2999:6,
      inlined from ‘btrfs_ioctl’ at fs/btrfs/ioctl.c:4616:10:
  ./include/linux/kasan-checks.h:38:27: warning: ‘space_args’ may be used
  uninitialized [-Wmaybe-uninitialized]
     38 | #define kasan_check_write __kasan_check_write
  ./include/linux/instrumented.h:129:9: note: in expansion of macro
  ‘kasan_check_write’
    129 |         kasan_check_write(to, n);
	|         ^~~~~~~~~~~~~~~~~
  ./include/linux/kasan-checks.h: In function ‘btrfs_ioctl’:
  ./include/linux/kasan-checks.h:20:6: note: by argument 1 of type ‘const
  volatile void *’ to ‘__kasan_check_write’ declared here
     20 | bool __kasan_check_write(const volatile void *p, unsigned int
	size);
	|      ^~~~~~~~~~~~~~~~~~~
  fs/btrfs/ioctl.c:2981:39: note: ‘space_args’ declared here
   2981 |         struct btrfs_ioctl_space_args space_args;
	|                                       ^~~~~~~~~~
  In function ‘instrument_copy_from_user_before’,
      inlined from ‘_copy_from_user’ at ./include/linux/uaccess.h:148:3,
      inlined from ‘copy_from_user’ at ./include/linux/uaccess.h:183:7,
      inlined from ‘_btrfs_ioctl_send’ at fs/btrfs/ioctl.c:4343:9,
      inlined from ‘btrfs_ioctl’ at fs/btrfs/ioctl.c:4658:10:
  ./include/linux/kasan-checks.h:38:27: warning: ‘args32’ may be used
  uninitialized [-Wmaybe-uninitialized]
     38 | #define kasan_check_write __kasan_check_write
  ./include/linux/instrumented.h:129:9: note: in expansion of macro
  ‘kasan_check_write’
    129 |         kasan_check_write(to, n);
	|         ^~~~~~~~~~~~~~~~~
  ./include/linux/kasan-checks.h: In function ‘btrfs_ioctl’:
  ./include/linux/kasan-checks.h:20:6: note: by argument 1 of type ‘const
  volatile void *’ to ‘__kasan_check_write’ declared here
     20 | bool __kasan_check_write(const volatile void *p, unsigned int
	size);
	|      ^~~~~~~~~~~~~~~~~~~
  fs/btrfs/ioctl.c:4341:49: note: ‘args32’ declared here
   4341 |                 struct btrfs_ioctl_send_args_32 args32;
	|                                                 ^~~~~~

This was due to his config options and having KASAN turned on,
which adds some extra checks around copy_from_user(), which then
triggered the -Wmaybe-uninitialized checker for these cases.

Fix the warnings by initializing the different structs we're copying
into.

Reported-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -2978,7 +2978,7 @@ static void get_block_group_info(struct list_head *groups_list,
 static long btrfs_ioctl_space_info(struct btrfs_fs_info *fs_info,
 				   void __user *arg)
 {
-	struct btrfs_ioctl_space_args space_args;
+	struct btrfs_ioctl_space_args space_args = { 0 };
 	struct btrfs_ioctl_space_info space;
 	struct btrfs_ioctl_space_info *dest;
 	struct btrfs_ioctl_space_info *dest_orig;
@@ -4338,7 +4338,7 @@ static int _btrfs_ioctl_send(struct inode *inode, void __user *argp, bool compat
 
 	if (compat) {
 #if defined(CONFIG_64BIT) && defined(CONFIG_COMPAT)
-		struct btrfs_ioctl_send_args_32 args32;
+		struct btrfs_ioctl_send_args_32 args32 = { 0 };
 
 		ret = copy_from_user(&args32, argp, sizeof(args32));
 		if (ret)