param: fix NULL comparison on oom

kp->arg is always true: it's the contents of that pointer we care about.
Reported-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: stable@kernel.org

kernel/params.c

@@ -222,7 +222,7 @@ int param_set_charp(const char *val, struct kernel_param *kp)
 	 * don't need to; this mangled commandline is preserved. */
 	if (slab_is_available()) {
 		*(char **)kp->arg = kstrdup(val, GFP_KERNEL);
-		if (!kp->arg)
+		if (!*(char **)kp->arg)
 			return -ENOMEM;
 	} else
 		*(const char **)kp->arg = val;