quota: Do not dirty bad dquots

Currently we mark dirty even dquots that are not active (i.e.,
initialization or reading failed for them). Thus later we have to check
whether dirty dquot is really active and just clear the dirty bit if
not. Avoid this complication by just never marking non-active dquot as
dirty.

Signed-off-by: Jan Kara <jack@suse.cz>