Fix tcp_lro_rx_ipv4() for drivers that do not set CSUM_IP_CHECKED. Specifcially, in_cksum_hdr() returns 0 (not 0xffff) when the IPv4 checksum is correct. Without this fix, the tcp_lro code will reject good IPv4 traffic from drivers that do not implement IPv4 header harder csum offload.
Fix tcp_lro_rx_ipv4() for drivers that do not set CSUM_IP_CHECKED.
Specifcially, in_cksum_hdr() returns 0 (not 0xffff) when the IPv4
checksum is correct. Without this fix, the tcp_lro code will reject
good IPv4 traffic from drivers that do not implement IPv4 header
harder csum offload.

Sponsored by: Myricom Inc.

MFC after:	7 days

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