cifs: fix potential double free during failed mount

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=2088799

Cc: stable@vger.kernel.org
Signed-off-by: Roberto Bergantinos <rbergant@redhat.com>
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/cifsfs.c

@@ -838,7 +838,7 @@ cifs_smb3_do_mount(struct file_system_type *fs_type,
 	      int flags, struct smb3_fs_context *old_ctx)
 {
 	int rc;
-	struct super_block *sb;
+	struct super_block *sb = NULL;
 	struct cifs_sb_info *cifs_sb = NULL;
 	struct cifs_mnt_data mnt_data;
 	struct dentry *root;
@@ -934,10 +934,12 @@ cifs_smb3_do_mount(struct file_system_type *fs_type,
 	return root;
 out:
 	if (cifs_sb) {
+		if (!sb || IS_ERR(sb)) {  /* otherwise kill_sb will handle */
 			kfree(cifs_sb->prepath);
 			smb3_cleanup_fs_context(cifs_sb->ctx);
 			kfree(cifs_sb);
 		}
+	}
 	return root;
 }