Clear transmit checksum offload context state upon lem(4) interface

initialization.  Prior to this change packets may be transmitted with an
incorrect checksum.

Em(4) already has an equivalent change in r213234.

Obtained From:  Sandvine
MFC After:      1 week
Approved by:    re (bz)

sys/dev/e1000/if_lem.c

@@ -2654,6 +2654,7 @@ lem_setup_transmit_structures(struct adapter *adapter)
 	}
 
 	/* Reset state */
+	adapter->last_hw_offload = 0;
 	adapter->next_avail_tx_desc = 0;
 	adapter->next_tx_to_clean = 0;
 	adapter->num_tx_desc_avail = adapter->num_tx_desc;