rgw_file: avoid a recursive lane lock in LRU drain

rgw_file: avoid a recursive lane lock in LRU drain

While highly unlikely in normal fs operations, draining an
entire filesystem could induce a recursion on object delete/unref,
when provoked by cohort_lru::drain().

Adjusted to use intrusive slist rather than std:vector<T*>
after review.

Found running librgw_file_nfsns unit test.

Fixes: http://tracker.ceph.com/issues/20374
Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>