g_file_set_contents(): use posix_fallocate()

Extents-based filesystems like knowing in advance how much data will be
written to a file in order to prevent fragmentation.  If we have it, use
posix_fallocate() before writing data in g_file_set_contents().

https://bugzilla.gnome.org/show_bug.cgi?id=701560

glib/gfileutils.c

@@ -1059,6 +1059,13 @@ write_to_temp_file (const gchar  *contents,
   if (length > 0)
     {
       gsize n_written;
+
+#ifdef HAVE_POSIX_FALLOCATE
+      /* We do this on a 'best effort' basis... It may not be supported
+       * on the underlying filesystem.
+       */
+      (void) posix_fallocate (fd, 0, length);
+#endif
       
       errno = 0;