quota: Forbid Q_GETQUOTA and Q_GETNEXTQUOTA for frozen filesystem

Commit 7955118eafc4 (quota: Allow Q_GETQUOTA for frozen filesystem)
allowed Q_GETQUOTA call for frozen filesystem. It makes sense on the
first look but zero-day testing has shown that with this change ext4
warns about starting a transaction for frozen filesystem. This happens
because ext4_acquire_dquot() prepares for allocating space for new quota
structure. Although it would be possible to implement Q_GETQUOTA for
ext4 without allocating space for non-existent structures, the matter
further complicates because OCFS2 needs to update on-disk structure use
count when a new cluster node loads quota information from disk. So just
revert the change and forbid Q_GETQUOTA together with Q_GETNEXTQUOTA for
frozen filesystem. Add comment to quotactl_cmd_write() to save us from
repeating this excercise in a few years when I forget again.

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

fs/quota/quota.c

@@ -765,11 +765,14 @@ static int do_quotactl(struct super_block *sb, int type, int cmd, qid_t id,
 /* Return 1 if 'cmd' will block on frozen filesystem */
 static int quotactl_cmd_write(int cmd)
 {
+	/*
+	 * We cannot allow Q_GETQUOTA and Q_GETNEXTQUOTA without write access
+	 * as dquot_acquire() may allocate space for new structure and OCFS2
+	 * needs to increment on-disk use count.
+	 */
 	switch (cmd) {
 	case Q_GETFMT:
 	case Q_GETINFO:
-	case Q_GETQUOTA:
-	case Q_GETNEXTQUOTA:
 	case Q_SYNC:
 	case Q_XGETQSTAT:
 	case Q_XGETQSTATV: