ext4: avoid calling dquot_get_next_id() if quota is not enabled

This should be fixed in the quota layer so we can test with the quota
mutex held, but for now, we need this to avoid tests from crashing the
kernel aborting the regression test suite.
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/super.c

@@ -1113,6 +1113,7 @@ static ssize_t ext4_quota_write(struct super_block *sb, int type,
 static int ext4_quota_enable(struct super_block *sb, int type, int format_id,
 			     unsigned int flags);
 static int ext4_enable_quotas(struct super_block *sb);
+static int ext4_get_next_id(struct super_block *sb, struct kqid *qid);
 
 static struct dquot **ext4_get_dquots(struct inode *inode)
 {
@@ -1129,7 +1130,7 @@ static const struct dquot_operations ext4_quota_operations = {
 	.alloc_dquot	= dquot_alloc,
 	.destroy_dquot	= dquot_destroy,
 	.get_projid	= ext4_get_projid,
-	.get_next_id	= dquot_get_next_id,
+	.get_next_id	= ext4_get_next_id,
 };
 
 static const struct quotactl_ops ext4_qctl_operations = {
@@ -5274,6 +5275,17 @@ static ssize_t ext4_quota_write(struct super_block *sb, int type,
 	return len;
 }
 
+static int ext4_get_next_id(struct super_block *sb, struct kqid *qid)
+{
+	const struct quota_format_ops	*ops;
+
+	if (!sb_has_quota_loaded(sb, qid->type))
+		return -ESRCH;
+	ops = sb_dqopt(sb)->ops[qid->type];
+	if (!ops || !ops->get_next_id)
+		return -ENOSYS;
+	return dquot_get_next_id(sb, qid);
+}
 #endif
 
 static struct dentry *ext4_mount(struct file_system_type *fs_type, int flags,