pam_systemd: Ignore vtnr when seat != seat0

logind considers it an error for a seat other than seat0 to have a
non-zero vtnr for CreateSession

src/login/pam-module.c

@@ -321,6 +321,12 @@ _public_ PAM_EXTERN int pam_sm_open_session(
                         get_seat_from_display(display, NULL, &vtnr);
         }
 
+        if (seat && !streq(seat, "seat0")) {
+                pam_syslog(handle, LOG_DEBUG,
+                      "Ignoring vtnr %d for %s which is not seat0", vtnr, seat);
+                vtnr = 0;
+        }
+
         if (!type)
                 type = !isempty(display) ? "x11" :
                         !isempty(tty) ? "tty" : "unspecified";