staging/lustre/libcfs: Fix kstrtouint return value check fix

Apparently kstrtouint could return not just -EINVAL, but also -ERANGE,
so make sure we just check the return value for something negative.

Noticed by Dan Carpenter <dan.carpenter@oracle.com>

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>