Fix bridging IPv6 fragments with pf reassembly.

Fix bridging IPv6 fragments with pf reassembly.

Sending IPv6 fragments over a bridge with pf did not work.  During
input pf reassembles the packet, and at bridge output it should be
refragmented.  This is only done for PF_FWD direction, but bridge(4)
and veb(4) called pf_test() with PF_OUT argument.

OK sashan@