Fix build errors due to CONFIG_BRANCH_TRACER=y

The code that enables branch tracing for all (non-constant) branches
plays games with the preprocessor and #define's the C 'if ()' construct
to do tracing.

That's all fine, but it fails for some unusual but valid C code that is
sometimes used in macros, notably by the intel-iommu code:

if (i=drhd->iommu, drhd->ignored) ..

because now the preprocessor complains about multiple arguments to the
'if' macro.

So make the macro expansion of this particularly horrid trick use
varargs, and handle the case of comma-expressions in if-statements. Use
another macro to do it cleanly in just one place.

This replaces a patch by David (and acked by Steven) that did this all
inside that one already-too-horrid macro.

Tested-by: Ingo Molnar <[e-mail]>
Cc: David Woodhouse <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/compiler.h

···
  * "Define 'is'", Bill Clinton
  * "Define 'if'", Steven Rostedt
  */
-#define if(cond) if (__builtin_constant_p((cond)) ? !!(cond) :		\
+#define if(cond, ...) __trace_if( (cond , ## __VA_ARGS__) )
+#define __trace_if(cond) \
+	if (__builtin_constant_p((cond)) ? !!(cond) :			\
 	({								\
 		int ______r;						\
 		static struct ftrace_branch_data			\