kernel: timer: Eliminate a race condition in expiration handling

When a timer is restarted from a high priority interrupt, it may
happen that the timer is re-added to the timeout list right after
it is removed from that list prior to execution of its expiration
handler but before that execution actually occurs. This leads to
an assertion failure reported for `z_add_timeout()` because then
that function, called from `z_timer_expiration_handler()` for
periodic timers, turns out to be adding a timeout that is already
added to the timeout list.
This commit detects such situation in `z_timer_expiration_handler()`
and makes that function exit immediately when that occurs (as the
timer was restared, its expiration handler should not be executed).

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>