rgw_file: interned RGWFileHandle objects need parent refs

rgw_file:  interned RGWFileHandle objects need parent refs

RGW NFS fhcache/RGWFileHandle operators assume existence of the
full chain of parents from any object to the its fs_root--this is
a consequence of the weakly-connected namespace design goal, and
not a defect.

This change ensures the invariant by taking a parent ref when
objects are interned (when a parent ref is guaranteed).  Parent
refs are returned when objects are destroyed--essentially by the
invariant, such a ref must exist.

The extra ref is omitted when parent->is_root(), as that node is
not in the LRU cache.

Fixes: http://tracker.ceph.com/issues/18650
Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>