gfs2: Add some missing quota locking

The quota code is missing some locking between local quota changes and
syncing those quota changes to the global quota file (gfs2_quota_sync);
in particular, qd->qd_change needs to be kept in sync with the
QDF_CHANGE change flag and the number of references held.  Use the
qd->qd_lockref.lock spinlock for that.

With the qd->qd_lockref.lock spinlock held, we can no longer call
lockref_get(), so turn qd_hold() into a variant that assumes that the
lock is held.  This function is really supposed to take an additional
reference when one or more references are already held, so check for
that instead of checking if the lockref is dead.

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