shared: add a single definition of libmount cleanup functions

Use a trivial header file to share mnt_free_tablep and mnt_free_iterp.
It would be nicer put this in mount-util.h, but libmount.h is not in the
default include path, and the build system would have to be adjusted to pass
pkg-config include path in various places, and it's just not worth the trouble.
A separate header file works nicely.

src/shared/libmount-util.h

@@ -0,0 +1,10 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+#pragma once
+
+/* This needs to be after sys/mount.h */
+#include <libmount.h>
+
+#include "macro.h"
+
+DEFINE_TRIVIAL_CLEANUP_FUNC(struct libmnt_table*, mnt_free_table);
+DEFINE_TRIVIAL_CLEANUP_FUNC(struct libmnt_iter*, mnt_free_iter);

src/shared/meson.build

@@ -95,6 +95,7 @@ shared_sources = files('''
         json-internal.h
         json.c
         json.h
+        libmount-util.h
         lockfile-util.c
         lockfile-util.h
         log-link.h