IB/hfi1: Add limit test for RC/UC send via loopback

Fix potential memory corruption and panic in loopback for IB_WR_SEND
variants.

The code blindly assumes the posted length will fit in the fetched rwqe,
which is not a valid assumption.

Fix by adding a limit test, and triggering the appropriate send completion
and putting the QP in an error state.  This mimics the handling for
non-loopback QPs.

Fixes: 15703461 ("IB/{hfi1, qib, rdmavt}: Move ruc_loopback to rdmavt")
Cc: <stable@vger.kernel.org> #v4.20+
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>