e1000: Make masks for TCP flag handling during TSO sysctl'able

[tuexen]

Summary

Add tso_tcp_flags_mask_first_segment, tso_tcp_flags_mask_middle_segment, and tso_tcp_flags_mask_last_segment sysctl-variables to control the handling of TCP flags during TSO.
This allows in particular to change the handling appropriate for classical ECN to an appropriate handling for accurate ECN.

[tuexen]

Include context.

[rscheff]

Can I ping @kib like this?

[kbowling]

Are you sure this can't be dealt with dynamically in em_txrx and igb_txrx? Admittedly I have no reason to spend a lot of time digging into this but my intuition is you can stuff the offsets into if_pkt_info_t in iflib.c and make the right decisions when constructing the TSO packet descriptor.

sys/dev/e1000/if_em.c

4951

The handler will run any time the sysctl tree is queried. I would gate the write to only apply on changes.

[kib]

sys/dev/e1000/if_em.c

4951

Isnt' the check for req->newptr already does that?