IB/rxe: Fix missing completion for mem_reg work requests

Run the completer task to post a work completion after processing
a memory registration or invalidate work request. This covers the
case where the memory registration or invalidate was the last work
request posted to the qp.

Signed-off-by: Vijay Immanuel <vijayi@attalasystems.com>
Reviewed-by: Yonatan Cohen <yonatanc@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>