datapath: Fix mutual exclusion with bridge on Linux 2.6.27+.

datapath: Fix mutual exclusion with bridge on Linux 2.6.27+.

Linux 2.6.27 introduces a new mechanism for sharing STP packets among
kernel modules, which means that the code in datapath.c to avoid loading
when the Linux bridging module is also loaded has false positives.  So
fall back on these newer kernels to a less reliable way of avoiding the
bridge module, but one that does not have false positives.

CC: Jean Tourrihles <jt@hpl.hp.com>