Instead of using simple_strtoul which "converts" invalid numbers to 0,
use strict_strtoul and perform error checking to ensure that userspace
passes us a valid unsigned long. This addresses problems with functions
such as writev, which might want to write a trailing newline -- the
newline should rightfully be rejected, but the value preceeding it
should be preserved.

Fixes BZ#46981.

Signed-off-by: Dave Reisner <dreisner@archlinux.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>