IB/rdmavt: Fix loopback send with invalidate ordering

The IBTA spec notes:

o9-5.2.1: For any HCA which supports SEND with Invalidate, upon receiving
an IETH, the Invalidate operation must not take place until after the
normal transport header validation checks have been successfully
completed.

The rdmavt loopback code does the validation after the invalidate.

Fix by relocating the operation specific logic for all SEND variants until
after the validity checks.

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>