kernel-doc: abstract filename mapping

Reduce duplication in follow-up work. No functional changes.
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

scripts/kernel-doc

@@ -2730,26 +2730,35 @@ sub local_unescape($) {
 	return $text;
 }
 
-sub process_file($) {
+sub map_filename($) {
     my $file;
-    my $identifier;
-    my $func;
-    my $descr;
-    my $in_purpose = 0;
-    my $initial_section_counter = $section_counter;
     my ($orig_file) = @_;
-    my $leading_space;
 
     if (defined($ENV{'SRCTREE'})) {
 	$file = "$ENV{'SRCTREE'}" . "/" . $orig_file;
-    }
-    else {
+    } else {
 	$file = $orig_file;
     }
+
     if (defined($source_map{$file})) {
 	$file = $source_map{$file};
     }
 
+    return $file;
+}
+
+sub process_file($) {
+    my $file;
+    my $identifier;
+    my $func;
+    my $descr;
+    my $in_purpose = 0;
+    my $initial_section_counter = $section_counter;
+    my ($orig_file) = @_;
+    my $leading_space;
+
+    $file = map_filename($orig_file);
+
     if (!open(IN,"<$file")) {
 	print STDERR "Error: Cannot open file $file\n";
 	++$errors;