ipvs: Fix race conditions in lblc scheduler

We can't access the cache entry outside of our critical read-locked region,
because someone may free that entry. And we also need to check in the critical
region wether the destination is still available, i.e. it's not in the trash.
If we drop our reference counter, the destination can be purged from the trash
at any time. Our caller only guarantees that no destination is moved to the
trash, while we are scheduling. Also there is no need for our own rwlock,
there is already one in the service structure for use in the schedulers.

Signed-off-by: Sven Wegener <sven.wegener@stealer.net>
Signed-off-by: Simon Horman <horms@verge.net.au>