linuxkpi: Fix mod_timer and del_timer_sync

mod_timer is supposed to return 1 if the modified timer was pending, which
is exactly what callout_reset does so return the value after checking
that it's a correct one in case the api change.
del_timer_sync returns int so add a function and handle that.

Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D24983