quota: Properly return errors from dquot_writeback_dquots()

Due to a switched left and right side of an assignment,
dquot_writeback_dquots() never returned error. This could result in
errors during quota writeback to not be reported to userspace properly.
Fix it.

CC: [e-mail]
Coverity-id: 1226884
Signed-off-by: Jan Kara <[e-mail]>

fs/quota/dquot.c

···
 			dqstats_inc(DQST_LOOKUPS);
 			err = sb->dq_op->write_dquot(dquot);
 			if (!ret && err)
-				err = ret;
+				ret = err;
 			dqput(dquot);
 			spin_lock(&dq_list_lock);
 		}