mds: trim null dentries proactively

mds: trim null dentries proactively

Instead of leaving null dentries (e.g. left
behind from unlinks) in the cache until they
fall out of the LRU, actively push them
to the bottom of the LRU and then consume
all nulls at the bottom in trim() even if
the cache is not oversized yet.

This fixes the case where standby replay daemons
would otherwise accumulate a cache full of
null dentries resulting from unlinks, and it
makes the behaviour of active daemons more
deterministic.

Fixes: http://tracker.ceph.com/issues/16919
Signed-off-by: John Spray <john.spray@redhat.com>