quota: Remove locking for writing to the old quota format

The old quota quota format has fixed offset in quota file based on ID so
there's no locking needed against concurrent modifications of the file
(locking against concurrent IO on the same dquot is still provided by
dq_lock).

Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Jan Kara <jack@suse.cz>