login: fix FDNAME in call to sd_pid_notify_with_fds()

$ git grep FDNAME
logind-session-device.c: ... "FDNAME=session-", sd->session->id);
logind-session-device.c: ... "FDNAME=session", sd->session->id);

Oops.

Fixes systemd#8343.  Or at least a more minimal reproducer.  Xorg still
dies when logind is restarted, but the Xorg message says this
is entirely deliberate.

(This could also be the reason I hit systemd#8035, instead of the race
condition I originally suggested).

(cherry picked from commit b5cdfa4)

[fbui: follow-up of 4050e47]

src/login/logind-session-device.c

@@ -513,7 +513,7 @@ int session_device_save(SessionDevice *sd) {
                 return 0;
 
         m = strjoina("FDSTORE=1\n"
-                     "FDNAME=session", sd->session->id);
+                     "FDNAME=session-", sd->session->id);
 
         r = sd_pid_notify_with_fds(0, false, m, &sd->fd, 1);
         if (r < 0)