udevadm-lock: switch things over to lock_generic_with_timeout()

udevadm-lock: switch things over to lock_generic_with_timeout()

This replaces the local implementation of a timeout file lock with our
new generic one.

Note that a comment in the old code claimed we couldn't use alarm()-like timeouts,
but htat's not entirely true: we can if we use SIGKILL, and thus know
for sure that the process will be dead in case the timer is hit before
we actually enter the file lock syscall. But we also know it will be
delivered if we hit after.