firstboot: use WRITE_STRING_FILE_LABEL more

Let's explicitly label 3 key files we generate in systemd-firstboot.

Addresses some more of #37857.

src/firstboot/firstboot.c

@@ -731,7 +731,7 @@ static int process_hostname(int rfd) {
                 return 0;
 
         r = write_string_file_at(pfd, f, arg_hostname,
-                                 WRITE_STRING_FILE_CREATE|WRITE_STRING_FILE_SYNC|WRITE_STRING_FILE_ATOMIC);
+                                 WRITE_STRING_FILE_CREATE|WRITE_STRING_FILE_SYNC|WRITE_STRING_FILE_ATOMIC|WRITE_STRING_FILE_LABEL);
         if (r < 0)
                 return log_error_errno(r, "Failed to write /etc/hostname: %m");
 
@@ -764,7 +764,7 @@ static int process_machine_id(int rfd) {
         }
 
         r = write_string_file_at(pfd, "machine-id", SD_ID128_TO_STRING(arg_machine_id),
-                                 WRITE_STRING_FILE_CREATE|WRITE_STRING_FILE_SYNC|WRITE_STRING_FILE_ATOMIC);
+                                 WRITE_STRING_FILE_CREATE|WRITE_STRING_FILE_SYNC|WRITE_STRING_FILE_ATOMIC|WRITE_STRING_FILE_LABEL);
         if (r < 0)
                 return log_error_errno(r, "Failed to write /etc/machine-id: %m");
 
@@ -1190,7 +1190,7 @@ static int process_kernel_cmdline(int rfd) {
         }
 
         r = write_string_file_at(pfd, "cmdline", arg_kernel_cmdline,
-                                 WRITE_STRING_FILE_CREATE|WRITE_STRING_FILE_SYNC|WRITE_STRING_FILE_ATOMIC);
+                                 WRITE_STRING_FILE_CREATE|WRITE_STRING_FILE_SYNC|WRITE_STRING_FILE_ATOMIC|WRITE_STRING_FILE_LABEL);
         if (r < 0)
                 return log_error_errno(r, "Failed to write /etc/kernel/cmdline: %m");