ocaml: Release runtime lock around call to nbd_close

OCaml 5 is stricter than earlier versions about correct locking.

We must release the OCaml runtime lock when calling nbd_close since it
may do some long-running operations and we want to allow concurrent
threads to run.

However specifically if there are callbacks (eg. a debug callback)
then we would end up trying to re-acquire the lock in the callback,
resulting in a crash:

  (gdb) bt
  #0  __pthread_kill_implementation (threadid=<optimized out>,
      signo=signo@entry=6, no_tid=no_tid@entry=0) at pthread_kill.c:44
  libguestfs#1  0x00007f47ffc8f773 in __pthread_kill_internal (signo=6,
      threadid=<optimized out>) at pthread_kill.c:78
  libguestfs#2  0x00007f47ffc3e71e in __GI_raise (sig=sig@entry=6)
      at ../sysdeps/posix/raise.c:26
  libguestfs#3  0x00007f47ffc2687f in __GI_abort () at abort.c:79
  libguestfs#4  0x0000560c9eb62779 in caml_fatal_error ()
  libguestfs#5  0x0000560c9eb63238 in caml_plat_fatal_error ()
  libguestfs#6  0x0000560c9eb4ea77 in caml_acquire_domain_lock ()
  libguestfs#7  0x0000560c9eb65cdc in caml_leave_blocking_section ()
  libguestfs#8  0x0000560c9eaf8a87 in debug_wrapper (user_data=0x560ca0af2670,
      context=0x7f47fff8ca60 "nbd_close", msg=0x560ca0af28b0 "closing handle")
      at ../nbd-c.c:187
  #9  0x00007f47fff7072f in nbd_internal_debug (h=h@entry=0x560ca0b57db0,
      context=0x7f47fff8ca60 "nbd_close", context@entry=0x0,
      fs=fs@entry=0x7f47fff8ca6a "closing handle")
      at /home/rjones/d/libnbd/lib/debug.c:90
  #10 0x00007f47fff73f23 in nbd_close (h=0x560ca0b57db0)
      at /home/rjones/d/libnbd/lib/handle.c:127
  #11 0x0000560c9eae8dbe in nbd_internal_ocaml_handle_finalize (
      hv=<optimized out>) at ../handle.c:39
  #12 nbd_internal_ocaml_nbd_close (hv=<optimized out>) at ../handle.c:62
  #13 <signal handler called>
  #14 0x0000560c9eae84dc in camlTest_140_explicit_close__entry () at NBD.ml:148
  #15 0x0000560c9eae5c5b in caml_program ()
  #16 <signal handler called>
  #17 0x0000560c9eb6cb77 in caml_startup_common ()
  #18 0x0000560c9eb6cbef in caml_main ()
  #19 0x0000560c9eae5910 in main ()

ocaml/handle.c

@@ -26,6 +26,7 @@
 #include <caml/fail.h>
 #include <caml/memory.h>
 #include <caml/mlvalues.h>
+#include <caml/threads.h>
 
 #include <libnbd.h>
 
@@ -36,7 +37,9 @@ nbd_internal_ocaml_handle_finalize (value hv)
 {
   struct nbd_handle *h = NBD_val (hv);
 
+  caml_enter_blocking_section ();
   nbd_close (h);
+  caml_leave_blocking_section ();
 }
 
 value