gfs2: separate holder for rgrps in gfs2_rename

Before this patch, gfs2_rename added a holder for the rgrp glock to
its array of holders, ghs. There's nothing wrong with that, but this
patch separates it into a separate holder. This is done to ensure
it's always locked last as per the proper glock lock ordering,
and also to pave the way for a future patch in which we will
lock the non-rgrp glocks asynchronously.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>