MFC r355580, r355673, and r357553.

r355580:
cxgbe(4): Use TX_PKTS2 work requests in netmap Tx if it's available.

TX_PKTS2 is more efficient within the firmware and this improves netmap
Tx by a few Mpps in some common scenarios.

r355673:
cxgbe(4): Never use hardware checksumming in netmap tx.

r357553:
cxgbe(4): Add a knob to allow netmap tx traffic to be checksummed by
the hardware.

hw.cxgbe.nm_txcsum=1

Sponsored by: Chelsio Communications