btrfs: handle memory allocation failure in btrfs_csum_one_bio

Since f8a53bb5 ("btrfs: handle checksum generation in the storage
layer") the failures of btrfs_csum_one_bio() are handled via
bio_end_io().

This means, we can return BLK_STS_RESOURCE from btrfs_csum_one_bio() in
case the allocation of the ordered sums fails.

This also fixes a syzkaller report, where injecting a failure into the
kvzalloc() call results in a BUG_ON().

Reported-by: syzbot+d8941552e21eac774778@syzkaller.appspotmail.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/file-item.c

@@ -792,7 +792,9 @@ blk_status_t btrfs_csum_one_bio(struct btrfs_bio *bbio)
 				sums = kvzalloc(btrfs_ordered_sum_size(fs_info,
 						      bytes_left), GFP_KERNEL);
 				memalloc_nofs_restore(nofs_flag);
-				BUG_ON(!sums); /* -ENOMEM */
+				if (!sums)
+					return BLK_STS_RESOURCE;
+
 				sums->len = bytes_left;
 				ordered = btrfs_lookup_ordered_extent(inode,
 								offset);