[TCP]: Abstract out all write queue operations.

This allows the write queue implementation to be changed,
for example, to one which allows fast interval searching.

Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/net/sock.h
Simple merge

diff --cc include/net/tcp.h
Simple merge

diff --cc net/ipv4/tcp.c
Simple merge

diff --cc net/ipv4/tcp_input.c
Simple merge

diff --cc net/ipv4/tcp_ipv4.c
Simple merge

diff --cc net/ipv4/tcp_output.c
Simple merge

diff --cc net/ipv4/tcp_timer.c
Simple merge