Redo LRU atomics streamlining change.

As "Amit Dror" <amitd@infinidat.com> pointed out, the lane mutex
linearization trick actually does impose a requirement for atomic
accesses to entry lru->refcount in other code paths.

Revert this change, but consolidate accesses where possible.  Improve
comments regarding invariants.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>
Display the client_id rec we found, not the, um, other one.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>