Btrfs: fix comparison in __btrfs_map_block()

Add missing comparison to op in expression, which was forgotten when doing
the REQ_OP transition.

Fixes: b3d3fa51 ("btrfs: update __btrfs_map_block for REQ_OP transition")
Signed-off-by: Vincent Stehlé <vincent.stehle@intel.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>