x86/refcounts: Switch to UD2 for exceptions

As done in commit 3b3a371c ("x86/debug: Use UD2 for WARN()"), this
switches to UD2 from UD0 to keep disassembly readable.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20180225165056.GA11719@beast