tcp: Increase tcp_sosend_agglim from 2 to 3

tcp: Increase tcp_sosend_agglim from 2 to 3

This change makes better use of TSO.

When 2 is used, most of the large TCP segments' size are 2*MSS or 3*MSS.
When 3 is used, most of the large TCP segments' size are 4*MSS or 5*MSS.

The math is quite simple, given 1448B segment size:
2 == 4096B == 2 segments
3 == 6144B == 4 segments

Increasing it to a higher value improves single full speed stream's CPU
utilization, but does not have much effect on multiple streams'.