IB/rdmavt: Set QP allowed opcodes after QP allocation

Currently QP allowed_ops is set after the QP is completely initialized.
This curtails the use of this optimization for any initialization before
allowed_ops is set.

Fix by adding a helper to determine the correct allowed_ops and moving the
setting of the allowed_ops to just after QP allocation.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>