btrfs: remove the sync_io flag in struct btrfs_bio_ctrl

The sync_io flag is equivalent to wbc->sync_mode == WB_SYNC_ALL, so
just check for that and remove the separate flag.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>