gfs2: Check quota consistency on mount

In gfs2_quota_init(), make sure that the per-node "quota_change%u" file
doesn't contain duplicate uids/gids.  Those duplicates would cause us to
acquire the glock corresponding to those ids repeatedly, which the glock
code doesn't allow.

When finding inconsistencies, we wipe them out and ignore them.  The
resulting quotas will likely be inconsistent, and running quotacheck(1)
is advised.

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