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

arch/x86/include/asm/refcount.h

@@ -17,7 +17,7 @@
 #define _REFCOUNT_EXCEPTION				\
 	".pushsection .text..refcount\n"		\
 	"111:\tlea %[counter], %%" _ASM_CX "\n"		\
-	"112:\t" ASM_UD0 "\n"				\
+	"112:\t" ASM_UD2 "\n"				\
 	ASM_UNREACHABLE					\
 	".popsection\n"					\
 	"113:\n"					\