x86/debug: Move kprobe_debug_handler() into exc_debug_kernel()

Kprobes are on kernel text, and thus only matter for #DB-from-kernel.
Kprobes are ordered before the generic notifier, preserve that order.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Daniel Thompson <daniel.thompson@linaro.org>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/r/20200902133200.847465360@infradead.org