checkpatch: warn when using extern with function prototypes in .h files

Using the extern keyword on function prototypes is superfluous visual
noise so suggest removing it.

Using extern can cause unnecessary line wrapping at 80 columns and
unnecessarily long multi-line function prototypes.
Signed-off-by: NJoe Perches <joe@perches.com>
Suggested-by: NHannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -3878,6 +3878,16 @@ sub process {
 			}
 		}
 
+# check for new externs in .h files.
+		if ($realfile =~ /\.h$/ &&
+		    $line =~ /^\+\s*(extern\s+)$Type\s*$Ident\s*\(/s) {
+			if (WARN("AVOID_EXTERNS",
+				 "extern prototypes should be avoided in .h files\n" . $herecurr) &&
+			    $fix) {
+				$fixed[$linenr - 1] =~ s/(.*)\bextern\b\s*(.*)/$1$2/;
+			}
+		}
+
 # check for new externs in .c files.
 		if ($realfile =~ /\.c$/ && defined $stat &&
 		    $stat =~ /^.\s*(?:extern\s+)?$Type\s+($Ident)(\s*)\(/s)