tools/ocaml/libs/{xb, mmap}: use Data_abstract_val wrapper

This is not strictly necessary since it is essentially a no-op
currently: a cast to void* and value*, even in OCaml 5.0.

However it does make it clearer that what we have here is not a regular
OCaml value, but one allocated with Abstract_tag or Custom_tag,
and follows the example from the manual more closely:
https://v2.ocaml.org/manual/intfc.html#ss:c-outside-head

It also makes it clearer that these modules have been reviewed for
compat with OCaml 5.0.

We cannot use OCaml finalizers here, because we want control over when to unmap
these pages from remote domains.
A follow-up commit will add use-after-free detection instead.

No functional change.

Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
---
Reason for inclusion in 4.17:
- make code follow best practice for upcoming OCaml 5.0 compiler (already in beta)

Changes since v2:
- add Acked-by line

Changes since v3:
- mention that use-after-free is fixed in another commit, and we cannot use
  finalizers here

Changes since v4:
- use Op_val

tools/ocaml/libs/mmap/mmap_stubs.h

@@ -30,4 +30,8 @@ struct mmap_interface
 	int len;
 };
 
+#ifndef Data_abstract_val
+#define Data_abstract_val(v) ((void*) Op_val(v))
+#endif
+
 #endif

tools/ocaml/libs/mmap/xenmmap_stubs.c

@@ -28,7 +28,7 @@
 #include <caml/fail.h>
 #include <caml/callback.h>
 
-#define Intf_val(a) ((struct mmap_interface *) a)
+#define Intf_val(a) ((struct mmap_interface *) Data_abstract_val(a))
 
 static int mmap_interface_init(struct mmap_interface *intf,
                                int fd, int pflag, int mflag,

tools/ocaml/libs/xb/xs_ring_stubs.c

@@ -35,7 +35,7 @@
 #include <sys/mman.h>
 #include "mmap_stubs.h"
 
-#define GET_C_STRUCT(a) ((struct mmap_interface *) a)
+#define GET_C_STRUCT(a) ((struct mmap_interface *) Data_abstract_val(a))
 
 /*
  * Bytes_val has been introduced by Ocaml 4.06.1. So define our own version