SLOW_WORK: Fix CIFS to pass THIS_MODULE to slow_work_register_user()

As of the patch:

	SLOW_WORK: Wait for outstanding work items belonging to a module to clear

	Wait for outstanding slow work items belonging to a module to clear
	when unregistering that module as a user of the facility.  This
	prevents the put_ref code of a work item from being taken away before
	it returns.

slow_work_register_user() takes a module pointer as an argument.  CIFS must now
pass THIS_MODULE as that argument, lest the following error be observed:

	fs/cifs/cifsfs.c: In function 'init_cifs':
	fs/cifs/cifsfs.c:1040: error: too few arguments to function 'slow_work_register_user'
Signed-off-by: David Howells <dhowells@redhat.com>

fs/cifs/cifsfs.c

@@ -1037,7 +1037,7 @@ init_cifs(void)
 	if (rc)
 		goto out_unregister_key_type;
 #endif
-	rc = slow_work_register_user();
+	rc = slow_work_register_user(THIS_MODULE);
 	if (rc)
 		goto out_unregister_resolver_key;