linuxkpi: Don't clobber result on failure

In kstrto*(), don't assign to *res until we know the conversion is
successful, and address issues that may result in warnings if code
that uses <linux/kstrtox.h> is compiled at high warning levels.

MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.
Reviewed by: bz, emaste
Differential Revision: https://reviews.freebsd.org/D54440