kgdb: Fix fall-through warning for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a
fall-through warning by explicitly adding a goto statement instead
of letting the code fall through to the next case.

Link: KSPP#115
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20210528200222.GA39201@embeddedor
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>

Author: GustavoARSilva

kernel/debug/debug_core.c

@@ -1038,6 +1038,7 @@ dbg_notify_reboot(struct notifier_block *this, unsigned long code, void *x)
 	switch (kgdbreboot) {
 	case 1:
 		kgdb_breakpoint();
+		goto done;
 	case -1:
 		goto done;
 	}