- Substitute sbdrop_internal() with sbcut_internal(). The latter doesn't free
  mbufs, but return chain of free mbufs to a caller. Caller can either reuse
  them or return to allocator in a batch manner.
- Implement sbdrop()/sbdrop_locked() as a wrapper around sbcut_internal().
- Expose sbcut_locked() for outside usage.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Approved by:	re (marius)
When processing ACK in tcp_do_segment, use sbcut_locked() instead of
sbdrop_locked() to cut acked mbufs from the socket buffer. Free this
chain a batch manner after the socket buffer lock is dropped.

This measurably reduces contention on socket buffer.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Approved by:	re (marius)