IB/cma: Add a missing rcu_read_unlock()

Ensure that validate_ipv4_net_dev() calls rcu_read_unlock() if
fib_lookup() fails. Detected by sparse. Compile-tested only.

Fixes: "IB/cma: Validate routing of incoming requests" (commit f887f2ac).
Cc: Haggai Eran <haggaie@mellanox.com>
Cc: stable <stable@vger.kernel.org>
Reviewed-by: NSagi Grimberg <sagig@mellanox.com>
Reviewed-by: NHaggai Eran <haggaie@mellanox.com>
Reviewed-by: NJason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: NDoug Ledford <dledford@redhat.com>

drivers/infiniband/core/cma.c

@@ -1126,10 +1126,7 @@ static bool validate_ipv4_net_dev(struct net_device *net_dev,
 
 	rcu_read_lock();
 	err = fib_lookup(dev_net(net_dev), &fl4, &res, 0);
-	if (err)
-		return false;
-
-	ret = FIB_RES_DEV(res) == net_dev;
+	ret = err == 0 && FIB_RES_DEV(res) == net_dev;
 	rcu_read_unlock();
 
 	return ret;