kgdb,debug-core,gdbstub: Hook the reboot notifier for debugger detach

The gdbstub and kdb should get detached if the system is rebooting.
Calling gdbstub_exit() will set the proper debug core state and send a
message to any debugger that is connected to correctly detach.

An attached debugger will receive the exit code from
include/linux/reboot.h based on SYS_HALT, SYS_REBOOT, etc...

Reported-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>

kernel/debug/gdbstub.c

@@ -1111,6 +1111,13 @@ void gdbstub_exit(int status)
 	unsigned char checksum, ch, buffer[3];
 	int loop;
 
+	if (!kgdb_connected)
+		return;
+	kgdb_connected = 0;
+
+	if (!dbg_io_ops || dbg_kdb_mode)
+		return;
+
 	buffer[0] = 'W';
 	buffer[1] = hex_asc_hi(status);
 	buffer[2] = hex_asc_lo(status);