net/smc: fix a NULL pointer dereference

In case alloc_ordered_workqueue fails, the fix returns NULL
to avoid NULL pointer dereference.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>