librgw: rgw_getattr refactor, RGWLibFS::stat_leaf (object)

librgw: rgw_getattr refactor, RGWLibFS::stat_leaf (object)

Rewrite rgw_getattr to just dispatch into RGWLibFS::getattr, where
we can better use RGWFileHandle and RGWLibFS internals.

Initially, this change intended to dispatch a new RGWStatObjRequest
in the file case, but during implementation, discovered that in
(apparently) all file cases, it has already been done--but the path
had stopped working with the naming change.

Current GETATTR_DIRS1 unit test, when repeated so relying on lookup
(rather than saving size after rgw_write/rgw_close), passes.

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