IB/rdmavt: Allow reserving just one qpn

qib needs to reserve only one qpn for non-verbs stuff. Also fixed the for
loop to reserve the end qpn.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Harish Chegondi <harish.chegondi@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>