fix mbuf leak if it does not fit in software queue

mbuf should be owned by if_transmit function in any case.

Submitted-by:   Andrew Rybchenko <Andrew.Rybchenko at oktetlabs.ru>
Sponsored by:   Solarflare Communications, Inc.

sys/dev/sfxge/sfxge_tx.c

@@ -536,6 +536,7 @@ sfxge_tx_packet_add(struct sfxge_txq *txq, struct mbuf *m)
 	return (0);
 
 fail:
+	m_freem(m);
 	return (rc);
 	
 }