btrfs: remove redundant csum buffer in btrfs_io_bio

The io_bio tracks checksums and has an inline buffer or an allocated
one. And there's a third member that points to the right one, but we
don't need to use an extra pointer for that. Let btrfs_io_bio::csum
point to the right buffer and check that the inline buffer is not
accidentally freed.

This shrinks struct btrfs_io_bio by 8 bytes.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/file-item.c

@@ -144,7 +144,10 @@ int btrfs_lookup_file_extent(struct btrfs_trans_handle *trans,
 
 static void btrfs_io_bio_endio_readpage(struct btrfs_io_bio *bio, int err)
 {
-	kfree(bio->csum_allocated);
+	if (bio->csum != bio->csum_inline) {
+		kfree(bio->csum);
+		bio->csum = NULL;
+	}
 }
 
 static blk_status_t __btrfs_lookup_bio_sums(struct inode *inode, struct bio *bio,
@@ -175,13 +178,12 @@ static blk_status_t __btrfs_lookup_bio_sums(struct inode *inode, struct bio *bio
 	nblocks = bio->bi_iter.bi_size >> inode->i_sb->s_blocksize_bits;
 	if (!dst) {
 		if (nblocks * csum_size > BTRFS_BIO_INLINE_CSUM_SIZE) {
-			btrfs_bio->csum_allocated = kmalloc_array(nblocks,
-					csum_size, GFP_NOFS);
-			if (!btrfs_bio->csum_allocated) {
+			btrfs_bio->csum = kmalloc_array(nblocks, csum_size,
+							GFP_NOFS);
+			if (!btrfs_bio->csum) {
 				btrfs_free_path(path);
 				return BLK_STS_RESOURCE;
 			}
-			btrfs_bio->csum = btrfs_bio->csum_allocated;
 			btrfs_bio->end_io = btrfs_io_bio_endio_readpage;
 		} else {
 			btrfs_bio->csum = btrfs_bio->csum_inline;

fs/btrfs/volumes.h

@@ -274,7 +274,6 @@ struct btrfs_io_bio {
 	u64 logical;
 	u8 *csum;
 	u8 csum_inline[BTRFS_BIO_INLINE_CSUM_SIZE];
-	u8 *csum_allocated;
 	btrfs_io_bio_end_io_t *end_io;
 	struct bvec_iter iter;
 	/*