Make Broadway work on freebsd

It seems that posix_fallocate gives an ENODEV error when
called on an fd opened with shm_open on freebsd. Fix up
the error check to only trigger if we get ENOSPC.

https://bugzilla.gnome.org/show_bug.cgi?id=742980

gdk/broadway/gdkbroadway-server.c

@@ -544,7 +544,7 @@ map_named_shm (char *name, gsize size)
 
 #ifdef HAVE_POSIX_FALLOCATE
   res = posix_fallocate (fd, 0, size);
-  if (res != 0)
+  if (res != 0 && errno == ENOSPC)
     {
       shm_unlink (name);
       g_error ("Not enough shared memory for window surface");