[TCP]: Tidy up skb_entail

Heck, it even saves us some few bytes:

[acme@newtoy net-2.6.20]$ codiff -f /tmp/tcp.o.before ../OUTPUT/qemu/net-2.6.20/net/ipv4/tcp.o
/pub/scm/linux/kernel/git/acme/net-2.6.20/net/ipv4/tcp.c:
  tcp_sendpage |   -7
  tcp_sendmsg  |   -5
 2 functions changed, 12 bytes removed
[acme@newtoy net-2.6.20]$

Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>