scripts: kernel-doc: fix troubles with line counts

There's currently a bug with the way kernel-doc script
counts line numbers that can be seen with:

	$ ./scripts/kernel-doc -rst  -enable-lineno include/linux/math64.h >all && ./scripts/kernel-doc -rst -internal -enable-lineno include/linux/math64.h >int && diff -U0 int all

	--- int	2020-09-28 12:58:08.927486808 +0200
	+++ all	2020-09-28 12:58:08.905486845 +0200
	@@ -1 +1 @@
	-#define LINENO 27
	+#define LINENO 26
	@@ -3 +3 @@
	-#define LINENO 16
	+#define LINENO 15
	@@ -9 +9 @@
	-#define LINENO 17
	+#define LINENO 16
	...

This is happening with perl version 5.30.3, but I'm not
so sure if this is a perl bug, or if this is due to something
else.

In any case, fixing it is easy. Basically, when "-internal"
parameter is used, the process_export_file() function opens the
handle "IN". This makes the line number to be incremented, as the
handler for the main open is also "IN".

Fix the problem by using a different handler for the
main open().

While here, add a missing close for it.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

scripts/kernel-doc

@@ -2268,7 +2268,7 @@ sub process_file($) {
 
     $file = map_filename($orig_file);
 
-    if (!open(IN,"<$file")) {
+    if (!open(IN_FILE,"<$file")) {
 	print STDERR "Error: Cannot open file $file\n";
 	++$errors;
 	return;
@@ -2277,9 +2277,9 @@ sub process_file($) {
     $. = 1;
 
     $section_counter = 0;
-    while (<IN>) {
+    while (<IN_FILE>) {
 	while (s/\\\s*$//) {
-	    $_ .= <IN>;
+	    $_ .= <IN_FILE>;
 	}
 	# Replace tabs by spaces
         while ($_ =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e) {};
@@ -2311,6 +2311,7 @@ sub process_file($) {
 	    print STDERR "${file}:1: warning: no structured comments found\n";
 	}
     }
+    close IN_FILE;
 }