It's -EFAULT, not -1 (and contrary to the comment in there,
__strnlen_user() can return 0 - on faults).

Cc: stable@vger.kernel.org
Acked-by: NRichard Kuo <rkuo@codeaurora.org>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>