kernel/sched: Correct locking in essential thread panic

Calling a (handled/ignored) panic with the scheduler lock held
produces spinlock errors in some circumstances, depending on whether
or not the swap gets reached before another context switch.  Release
the lock around the call, we don't touch any scheduler state on the
path to z_swap(), so this is safe.

Signed-off-by: Andy Ross <andyross@google.com>