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>