9p v9fs_parse_options: replace simple_strtoul with kstrtouint

This is also a checkpatch change, but this one might have more implications
so keeping this separate

Link: http://lkml.kernel.org/r/20211102134608.1588018-4-dominique.martinet@atmark-techno.com
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>