sched: Add wait/wake interface for variable updated under a lock.

sched: Add wait/wake interface for variable updated under a lock.

Sometimes we need to wait for a condition to be true which must be
testing while holding a lock.  Correspondingly the condition is made
true while holding the lock and the wake up is sent under the lock.

This patch provides wake and wait interfaces which can be used for this
situation when the lock is a mutex or a spinlock, or any other lock for
which there are foo_lock() and foo_unlock() functions.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240925053405.3960701-6-neilb@suse.de