cstyle: understand macro params can be empty

It's not uncommon to have empty parameters in code generator macros,
usually when multiple parameters are concatenated or stringified into a
single token or literal. So, exclude the space-before-comma check, which
will allow construction like `MACRO_CALL(foo, , baz)`.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #16840

scripts/cstyle.pl

@@ -572,7 +572,9 @@ ($$)
 		err("comma or semicolon followed by non-blank");
 	}
 	# allow "for" statements to have empty "while" clauses
-	if (/\s[,;]/ && !/^[\t]+;$/ && !/^\s*for \([^;]*; ;[^;]*\)/) {
+	# allow macro invocations to have empty parameters
+	if (/\s[,;]/ && !/^[\t]+;$/ &&
+	    !($in_macro_call || /^\s*for \([^;]*; ;[^;]*\)/)) {
 		err("comma or semicolon preceded by blank");
 	}
 	if (/^\s*(&&|\|\|)/) {