btrfs: Remove btrfs_bio::flags member

The last user of btrfs_bio::flags was removed in commit 326e1dbb
("block: remove management of bi_remaining when restoring original
bi_end_io"), remove it.

(Tagged for stable as the structure is heavily used and space savings
are desirable.)

CC: stable@vger.kernel.org # 4.4+
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.h

@@ -319,7 +319,6 @@ struct btrfs_bio {
 	u64 map_type; /* get from map_lookup->type */
 	bio_end_io_t *end_io;
 	struct bio *orig_bio;
-	unsigned long flags;
 	void *private;
 	atomic_t error;
 	int max_errors;