printk: Collapse shutdown types into a single dump reason

To turn the KMSG_DUMP_* reasons into a more ordered list, collapse
the redundant KMSG_DUMP_(RESTART|HALT|POWEROFF) reasons into
KMSG_DUMP_SHUTDOWN. The current users already don't meaningfully
distinguish between them, so there's no need to, as discussed here:
https://lore.kernel.org/lkml/CA+CK2bAPv5u1ih5y9t5FUnTyximtFCtDYXJCpuyjOyHNOkRdqw@mail.gmail.com/

Link: https://lore.kernel.org/lkml/20200515184434.8470-2-keescook@chromium.org/
Reviewed-by: Pavel Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Signed-off-by: Kees Cook <keescook@chromium.org>

include/linux/kmsg_dump.h

@@ -25,9 +25,7 @@ enum kmsg_dump_reason {
 	KMSG_DUMP_PANIC,
 	KMSG_DUMP_OOPS,
 	KMSG_DUMP_EMERG,
-	KMSG_DUMP_RESTART,
-	KMSG_DUMP_HALT,
-	KMSG_DUMP_POWEROFF,
+	KMSG_DUMP_SHUTDOWN,
 };
 
 /**