futex: request only one page from get_user_pages()

Yanmin noticed that fault_in_user_writeable() requests 4 pages instead
of one.

That's the result of blindly trusting Linus' proposal :) I even looked
up the prototype to verify the correctness: the argument in question
is confusingly enough named "len" while in reality it means number of
pages.
Pointed-out-by: NYanmin Zhang <yanmin_zhang@linux.intel.com>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>

kernel/futex.c

@@ -299,7 +299,7 @@ void put_futex_key(int fshared, union futex_key *key)
 static int fault_in_user_writeable(u32 __user *uaddr)
 {
 	int ret = get_user_pages(current, current->mm, (unsigned long)uaddr,
-				 sizeof(*uaddr), 1, 0, NULL, NULL);
+				 1, 1, 0, NULL, NULL);
 	return ret < 0 ? ret : 0;
 }