rgw_file: RGWFileHandle dtor must also cond-unlink from FHCache

rgw_file: RGWFileHandle dtor must also cond-unlink from FHCache

Formerly masked in part by the reclaim() action, direct-delete now
substitutes for reclaim() iff its LRU lane is over its high-water
mark, and in particular, like reclaim() the destructor is certain
to see handles still interned on the FHcache when nfs-ganesha is
recycling objects from its own LRU.

Fixes: http://tracker.ceph.com/issues/19112
Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>
(cherry picked from commit d51a3b1224ba62bb53c6c2c7751fcf7853c35a4b)