btrfs: refactor allocation size calculation in alloc_btrfs_io_context()

Use struct_size() to replace the open-coded calculation, remove the
comment as use of the helper is self explanatory.

Reviewed-by: Qu Wenruo <[email protected]>
Signed-off-by: Mehdi Ben Hadj Khelifa <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: Mehdi Ben Hadj Khelifa

fs/btrfs/volumes.c

@@ -6076,12 +6076,7 @@ struct btrfs_io_context *alloc_btrfs_io_context(struct btrfs_fs_info *fs_info,
 {
 	struct btrfs_io_context *bioc;
 
-	bioc = kzalloc(
-		 /* The size of btrfs_io_context */
-		sizeof(struct btrfs_io_context) +
-		/* Plus the variable array for the stripes */
-		sizeof(struct btrfs_io_stripe) * (total_stripes),
-		GFP_NOFS);
+	bioc = kzalloc(struct_size(bioc, stripes, total_stripes), GFP_NOFS);
 
 	if (!bioc)
 		return NULL;