btrfs: fix double-free 'tree_root' in 'btrfs_mount()'

On error path 'tree_root' is treed in 'free_fs_info()'.
No need to free it explicitely. Noticed by SLUB in debug mode:

Complete reproducer under usermode linux (discovered on real
machine):

    bdev=/dev/ubda
    btr_root=/btr
    /mkfs.btrfs $bdev
    mount $bdev $btr_root
    mkdir $btr_root/subvols/
    cd $btr_root/subvols/
    /btrfs su cr foo
    /btrfs su cr bar
    mount $bdev -osubvol=subvols/foo $btr_root/subvols/bar
    umount $btr_root/subvols/bar

which gives

device fsid 4d55aa28-45b1-474b-b4ec-da912322195e devid 1 transid 7 /dev/ubda
=============================================================================
BUG kmalloc-2048: Object already free
-----------------------------------------------------------------------------

INFO: Allocated in btrfs_mount+0x389/0x7f0 age=0 cpu=0 pid=277
INFO: Freed in btrfs_mount+0x51c/0x7f0 age=0 cpu=0 pid=277
INFO: Slab 0x0000000062886200 objects=15 used=9 fp=0x0000000070b4d2d0 flags=0x4081
INFO: Object 0x0000000070b4d2d0 @offset=21200 fp=0x0000000070b4a968
...
Call Trace:
70b31948:  [<6008c522>] print_trailer+0xe2/0x130
70b31978:  [<6008c5aa>] object_err+0x3a/0x50
70b319a8:  [<6008e242>] free_debug_processing+0x142/0x2a0
70b319e0:  [<600ebf6f>] btrfs_mount+0x55f/0x7f0
70b319f8:  [<6008e5c1>] __slab_free+0x221/0x2d0

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Cc: Arne Jansen <sensille@gmx.net>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/super.c

@@ -933,8 +933,12 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
 	 * then open_ctree will properly initialize everything later.
 	 */
 	fs_info = kzalloc(sizeof(struct btrfs_fs_info), GFP_NOFS);
+	if (!fs_info) {
+		error = -ENOMEM;
+		goto error_close_devices;
+	}
 	tree_root = kzalloc(sizeof(struct btrfs_root), GFP_NOFS);
-	if (!fs_info || !tree_root) {
+	if (!tree_root) {
 		error = -ENOMEM;
 		goto error_close_devices;
 	}
@@ -964,7 +968,6 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
 
 		btrfs_close_devices(fs_devices);
 		free_fs_info(fs_info);
-		kfree(tree_root);
 	} else {
 		char b[BDEVNAME_SIZE];
 
@@ -992,7 +995,6 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
 error_close_devices:
 	btrfs_close_devices(fs_devices);
 	free_fs_info(fs_info);
-	kfree(tree_root);
 	return ERR_PTR(error);
 }