mm, percpu: Make sure percpu_alloc early parameter has an argument

Otherwise we are getting a nil dereference if percpu_alloc kernel boot
argument is specified without value.

 | [    0.000000] BUG: unable to handle kernel NULL pointer dereference at           (null)
 | [    0.000000] IP: [<ffffffff81391360>] strcmp+0x10/0x30
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

mm/percpu.c

@@ -1380,6 +1380,9 @@ enum pcpu_fc pcpu_chosen_fc __initdata = PCPU_FC_AUTO;
 
 static int __init percpu_alloc_setup(char *str)
 {
+	if (!str)
+		return -EINVAL;
+
 	if (0)
 		/* nada */;
 #ifdef CONFIG_NEED_PER_CPU_EMBED_FIRST_CHUNK