btrfs: drop extent_io_ops::set_range_writeback callback

The data and metadata callback implementation both use the same
function. We can remove the call indirection and intermediate helper
completely.

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

fs/btrfs/disk-io.c

@@ -4528,7 +4528,6 @@ static const struct extent_io_ops btree_extent_io_ops = {
 	.submit_bio_hook = btree_submit_bio_hook,
 	.readpage_end_io_hook = btree_readpage_end_io_hook,
 	.readpage_io_failed_hook = btree_io_failed_hook,
-	.set_range_writeback = btrfs_set_range_writeback,
 
 	/* optional callbacks */
 };