btrfs: Remove btrfs_bio::flags member

commit 34b127aecd4fe8e6a3903e10f204a7b7ffddca22 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>