locking: Handle spurious notifications on lock acquire

This was already a TODO since the implementation of lock priorities in
September 2010. Under certain conditions a waiting acquire can be
notified at a time when it can't actually get the lock. In this case it
would try and fail to acquire the lock and then return to the caller
before the timeout ends.

While this is not bad (nothing breaks), it isn't nice either. A separate
patch will prevent unnecessary notifications when shared locks are
released.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>