memcg_write_event_control(): fix a user-triggerable oops

we are *not* guaranteed that anything past the terminating NUL
is mapped (let alone initialized with anything sane).

Fixes: 0dea1168 ("cgroup: implement eventfd-based generic API for notifications")
Cc: stable@vger.kernel.org
Cc: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

mm/memcontrol-v1.c

@@ -1842,9 +1842,12 @@ static ssize_t memcg_write_event_control(struct kernfs_open_file *of,
 	buf = endp + 1;
 
 	cfd = simple_strtoul(buf, &endp, 10);
-	if ((*endp != ' ') && (*endp != '\0'))
-		return -EINVAL;
+	if (*endp == '\0')
+		buf = endp;
+	else if (*endp == ' ')
 		buf = endp + 1;
+	else
+		return -EINVAL;
 
 	event = kzalloc(sizeof(*event), GFP_KERNEL);
 	if (!event)