gfs2: Add a crc field to resource group headers

Add the rg_crc field to store a crc32 of the gfs2_rgrp structure. This
allows us to check resource group headers' integrity and removes the
requirement to check them against the rindex entries in fsck. If this
field is found to be zero, it should be ignored (or updated with an
accurate value).

Signed-off-by: Andrew Price <anprice@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>