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: Yanmin Zhang <yanmin_zhang@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>