svcrdma: Fence LOCAL_INV work requests

Fencing forces the invalidate to only happen after all prior send
work requests have been completed.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Reported by : Devesh Sharma <Devesh.Sharma@Emulex.Com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>