sched: ast_sched_del may return prematurely due to spurious wakeup

When deleting a scheduled item if the item in question is currently
executing the ast_sched_del function waits until it has completed.
This is accomplished using ast_cond_wait. Unfortunately the
ast_cond_wait function can suffer from spurious wakeups so the
predicate needs to be checked after it returns to make sure it has
really woken up as a result of being signaled.

This change adds a loop around the ast_cond_wait to make sure that
it only exits when the executing task has really completed.

ASTERISK-25355 #close

Change-Id: I51198270eb0b637c956c61aa409f46283432be61