[GFS2] Don't copy meta_header for rgrp in and out

The meta_header for an ondisk rgrp never changes, so there is no point
copying it in and back out to disk. Also there is no reason to keep
a copy for each rgrp in memory.

The code already checks to ensure that the header is correct before
it calls the routine to copy the data in, so that we don't even need
to check whether its correct on disk in the functions in ondisk.c

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>