staging/rdma/hfi1: close shared context security hole

Driver does not verify userid for shared context assignments, allowing
malicious user access.

Reviewed by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Jareer H Abdel-Qader <jareer.h.abdel-qader@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>