MFS r353182:
Make sure the transmit loop doesn't get starved in ipoib.

When the software send queue gets filled up, callbacks to
if_transmit will stop. Make sure the transmit callback
routine checks the send queue and outputs any remaining
mbufs. Else the remaining mbufs may simply sit in the
output queue blocking the transmit path.

Sponsored by: Mellanox Technologies
Approved by: re (gjb)