IB/qib: RCU locking for MR validation

Profiling indicates that MR validation locking is expensive.  The MR
table is largely read-only and is a suitable candidate for RCU locking.

The patch uses RCU locking during validation to eliminate one
lock/unlock during that validation.

Reviewed-by: Mike Heinz <michael.william.heinz@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/infiniband/hw/qib/qib_mr.c

@@ -527,3 +527,10 @@ int qib_dealloc_fmr(struct ib_fmr *ibfmr)
 out:
 	return ret;
 }
+
+void mr_rcu_callback(struct rcu_head *list)
+{
+	struct qib_mregion *mr = container_of(list, struct qib_mregion, list);
+
+	complete(&mr->comp);
+}