ext4: Disable dirty list tracking of dquots when journalling quotas

When journalling quotas, we writeback all dquots immediately after
changing them as part of current transation. Thus there's no need to
write anything in dquot_writeback_dquots() and so we can avoid updating
list of dirty dquots to reduce dq_list_lock contention.

This change reduces time to create 500000 files on ext4 on ramdisk by 50
different processes in separate directories by 15% when user quota is
turned on.
Signed-off-by: Jan Kara <jack@suse.cz>

fs/ext4/super.c

@@ -5404,6 +5404,13 @@ static int ext4_quota_on(struct super_block *sb, int type, int format_id,
 			ext4_msg(sb, KERN_WARNING,
 				"Quota file not on filesystem root. "
 				"Journaled quota will not work");
+		sb_dqopt(sb)->flags |= DQUOT_NOLIST_DIRTY;
+	} else {
+		/*
+		 * Clear the flag just in case mount options changed since
+		 * last time.
+		 */
+		sb_dqopt(sb)->flags &= ~DQUOT_NOLIST_DIRTY;
 	}
 
 	/*
@@ -5500,7 +5507,7 @@ static int ext4_enable_quotas(struct super_block *sb)
 		test_opt(sb, PRJQUOTA),
 	};
 
-	sb_dqopt(sb)->flags |= DQUOT_QUOTA_SYS_FILE;
+	sb_dqopt(sb)->flags |= DQUOT_QUOTA_SYS_FILE | DQUOT_NOLIST_DIRTY;
 	for (type = 0; type < EXT4_MAXQUOTAS; type++) {
 		if (qf_inums[type]) {
 			err = ext4_quota_enable(sb, type, QFMT_VFS_V1,