RDMA/rxe: Fix incorrect responder length checking

The code in rxe_resp.c at check_length() is incorrect as it compares
pkt->opcode an 8 bit value against various mask bits which are all higher
than 256 so nothing is ever reported.

This patch rewrites this to compare against pkt->mask which is
correct. However this now exposes another error. For UD send packets the
value of the pmtu cannot be determined from qp->mtu.  All that is required
here is to later check if the payload fits into the posted receive buffer
in that case.

Fixes: 837a5584

 ("RDMA/rxe: Implement packet length validation on responder")
Link: https://lore.kernel.org/r/20221208210945.28607-1-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Reviewed-by: Daisuke Matsuda <matsuda-daisuke@fujitsu.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>