gfs2: Stop messing with ip->i_rgd in the rlist code

In the resource group list code, keep the last resource group added in
the last position in the array.  Check against that instead of messing
with ip->i_rgd.

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