quota: add supprt for gfs2

quota: add supprt for gfs2

gfs2 uses the same generic quota interface as xfs and it has the same base
block/quota block size ratio and seems to work nice with the xfs quota module.

(People using gfs should be aware that quota reporting is lagging quite a bit
on gfs.  If you copy a file on a gfs volume the quota values are being updated
with a delay of 30s here with kernel 3.5. This reporting can lead to data
corruption if a client thinks he can write but actually he suddently can't.)