cifs: fix crash in cifs_dfs_do_automount

RHBZ: 1649907

Fix a crash that happens while attempting to mount a DFS referral from the same server on the root of a filesystem.
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/connect.c

@@ -4484,11 +4484,13 @@ cifs_are_all_path_components_accessible(struct TCP_Server_Info *server,
 					unsigned int xid,
 					struct cifs_tcon *tcon,
 					struct cifs_sb_info *cifs_sb,
-					char *full_path)
+					char *full_path,
+					int added_treename)
 {
 	int rc;
 	char *s;
 	char sep, tmp;
+	int skip = added_treename ? 1 : 0;
 
 	sep = CIFS_DIR_SEP(cifs_sb);
 	s = full_path;
@@ -4503,7 +4505,14 @@ cifs_are_all_path_components_accessible(struct TCP_Server_Info *server,
 		/* next separator */
 		while (*s && *s != sep)
 			s++;
-
+		/*
+		 * if the treename is added, we then have to skip the first
+		 * part within the separators
+		 */
+		if (skip) {
+			skip = 0;
+			continue;
+		}
 		/*
 		 * temporarily null-terminate the path at the end of
 		 * the current component
@@ -4551,8 +4560,7 @@ static int is_path_remote(struct cifs_sb_info *cifs_sb, struct smb_vol *vol,
 
 	if (rc != -EREMOTE) {
 		rc = cifs_are_all_path_components_accessible(server, xid, tcon,
-							     cifs_sb,
-							     full_path);
+			cifs_sb, full_path, tcon->Flags & SMB_SHARE_IS_IN_DFS);
 		if (rc != 0) {
 			cifs_dbg(VFS, "cannot query dirs between root and final path, "
 				 "enabling CIFS_MOUNT_USE_PREFIX_PATH\n");