IB/rdmavt: Compress adjacent SGEs in rvt_lkey_ok()

SGEs that are contiguous needlessly consume driver dependent TX resources.

The lkey validation logic is enhanced to compress the SGE that ends
up in the send wqe when consecutive addresses are detected.

The lkey validation API used to return 1 (success) or 0 (fail).

The return value is now an -errno, 0 (compressed), or 1 (uncompressed).  A
additional argument is added to pass the last SQE for the compression.

Loopback callers always pass a NULL to last_sge since the optimization is
of little benefit in that situation.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Brian Welty <brian.welty@intel.com>
Signed-off-by: Venkata Sandeep Dhanalakota <venkata.s.dhanalakota@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>