kernel: events: fix walking the waitq race condition

Fixes race condition for k_event_post_internal() in an
SMP environment while walking the waitq. Uses z_sched_waitq_walk()
to safely walk the waitq by using a sched_spinlock.
It should be noted that since walking the wait queue is an
operation of indeterminant length, there exists the possibility
that the sched_spinlock (which is a highly used and contended-for
lock) may be locked for an indeterminant amount of time. However,
it is expected that few threads will be waiting on any given kernel
event object, which should ameliorate this risk.

Fixes #54317

Signed-off-by: Aastha Grover <aastha.grover@intel.com>