s3/locking: Fix comments to reflect code flow

s3/locking: Fix comments to reflect code flow

* posix_pending_close_db is an in-memory database and not a tdb.
  Therefore adjusting comments to convey the correct meaning of the
  database.
* we do not have posix_locking_close_file() any more which got renamed
  to locking_close_file(). Thus fixing comment to mention the new name.

Signed-off-by: Anoop C S <anoopcs@redhat.com>
Reviewed-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Wed Sep 19 22:23:28 CEST 2018 on sn-devel-144