Improve use of socket buffer upcalls.

Use soreadable()/sowriteable() in socket upcalls to avoid extra wakeups
until we have enough data to read or space to write.

Increase partial receive len from 1K to 128K to not wake up on every
received packet.

This significantly reduces locks congestion and CPU usage and improves
throughput for large I/Os on NICs without TSO and LRO.

Reviewed by:	trasz
Sponsored by:	iXsystems, Inc.
mdoc and spelling cleanup.

MFC after:	3 days
Update the llvm/clang patch for r264345.