CDRIVER-4627 use posix_fallocate when available (#1244)

Avoids an unexpected crash with SIGBUS when trying to allocate a /dev/shm shared memory but there is not enough memory. 

Indeed ftruncate doesn't ensure memory is properly allocated, it ony
changes the file size from the VFS point of view, but doesn't actually
allocate any memory. So ftruncate might work despite we have no memory
left, and later when trying to zero-memset the mmapped buffer, we might
actually get a SIGBUS signal crashing the whole process.

Instead, make sure we can allocate the whole shared memory using
posix_fallocate and gracefully handle allocation problems, without
crashing.

The chromium project faced a similar issue in the past:
https://bugs.chromium.org/p/chromium/issues/detail?id=951431

Author: Romain-Geissler-1A

src/libmongoc/src/mongoc/mongoc-counters.c

@@ -181,15 +181,18 @@ mongoc_counters_alloc (size_t size)
       goto fail_noclean;
    }
 
-   /*
-    * NOTE:
-    *
-    * ftruncate() will cause reads to be zero. Therefore, we don't need to
-    * do write() of zeroes to initialize the shared memory area.
-    */
+#if defined(__APPLE__)
+   // macOS does not have posix_fallocate available.
    if (-1 == ftruncate (fd, size)) {
       goto fail_cleanup;
    }
+#else
+   // Prefer posix_fallocate on Linux. posix_fallocate ensures that disk space
+   // is allocated.
+   if (0 != posix_fallocate (fd, 0, size)) {
+      goto fail_cleanup;
+   }
+#endif // __APPLE__
 
    mem = mmap (NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    if (mem == MAP_FAILED) {