IB/rdmavt: Add IB user context allocation and de-alloction functions

Adding IB user context alloc and dealloc functions to rdmavt so that the
drivers that use rdmavt can use these functions instead of defining their
own functions.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Harish Chegondi <harish.chegondi@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>