hpfs: Replace simple_strtoul with kstrtoint in hpfs_parse_param

kstrtoint() is better because simple_strtoul() ignores overflow and the
type of 'timeshift' is 'int' rather than 'unsigned long'. Using kstrtoint()
is more concise too.

Signed-off-by: Su Hui <suhui@nfschina.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>