Btrfs: use btrfs_op instead of bio_op in __btrfs_map_block

This seems to be a leftover of commit cf8cddd3 ("btrfs: don't
abuse REQ_OP_* flags for btrfs_map_block").

It should use btrfs_op() helper to provide one of 'enum btrfs_map_op'
types.

Fixes: cf8cddd3 ("btrfs: don't abuse REQ_OP_* flags for btrfs_map_block")
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: Satoru Takeuchi <satoru.takeuchi@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>