Fix a couple of problems that may make gem(4) get stuck:

Fix a couple of problems that may make gem(4) get stuck:

1. If bus_dmamap_load_mbuf() fails because there are not enough
   segments in the map, defrag the mbuf.

2. If there are not enough free (hardware ring) descriptors, set
   IFF_OACTIVE and keep the packet on the queue.

3. If there is some other resource starvation that makes
   bus_dmamap_load_mbuf() or defragmentation fail, drop the packet.
   Don't set IFF_OACTIVE, since the Tx ring could be empty and we'd be
   stuck.

4. Only pass packets that are actually handed off to the hardware to
   BPF.  Do so before handing them off to the hardware to make sure
   the packet isn't freed behind our back.

ok dlg@