rgw_file: refcnt bugfixes

rgw_file: refcnt bugfixes

This change includes 3 related changes:

1. add required lock flags for FHCache updates--this is a crash
   bug under concurrent update/lookup

2. omit to inc/dec refcnt on root filehandles in 2 places--the
   root handle current is not on the lru list, so it's not
   valid to do so

3. based on observation of LRU behavior during creates/deletes,
   update (cohort) LRU unref to move objects to LRU when their
   refcount falls to SENTINEL_REFCNT--this cheaply primes the
   current reclaim() mechanism, so very significanty improves
   space use (e.g., after deletes) in the absence of scans
   (which is common due to nfs-ganesha caching)

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>