checkpatch: improve "no space is necessary after a cast" test

The "no space is necessary after a cast" sizeof exclusion doesn't work
properly.

The test reports a false positive for code like:

	BUILD_BUG_ON(sizeof(struct batadv_bla_claim_dst) != 6);

Make it work, simplify the exclusions, and add some comments.
Signed-off-by: Joe Perches <joe@perches.com>
Reported-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -2552,8 +2552,15 @@ sub process {
 			}
 		}
 
-		if ($line =~ /^\+.*(\w+\s*)?\(\s*$Type\s*\)[ \t]+(?!$Assignment|$Arithmetic|[,;:\?\(\{\}\[\<\>]|&&|\|\||\\$)/ &&
-		    (!defined($1) || $1 !~ /sizeof\s*/)) {
+# check for space after cast like "(int) foo" or "(struct foo) bar"
+# avoid checking a few false positives:
+#   "sizeof(<type>)" or "__alignof__(<type>)"
+#   function pointer declarations like "(*foo)(int) = bar;"
+#   structure definitions like "(struct foo) { 0 };"
+#   multiline macros that define functions
+#   known attributes or the __attribute__ keyword
+		if ($line =~ /^\+(.*)\(\s*$Type\s*\)([ \t]++)((?![={]|\\$|$Attribute|__attribute__))/ &&
+		    (!defined($1) || $1 !~ /\b(?:sizeof|__alignof__)\s*$/)) {
 			if (CHK("SPACING",
 				"No space is necessary after a cast\n" . $herecurr) &&
 			    $fix) {