RDMA/core: Unify RoCE check and re-factor code

In cm_req_handler(), unify the check for RoCE and re-factor to avoid
one test.

Link: https://lore.kernel.org/r/1617705423-15570-1-git-send-email-haakon.bugge@oracle.com


Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Fixes: 8f974860 ("IB/cm: Reduce dependency on gid attribute ndev check")
Fixes: 194f64a3 ("RDMA/core: Fix corrupted SL on passive side")
Signed-off-by: Håkon Bugge <haakon.bugge@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>