cleaning up struct sock

Hi,

This one turns IP_SK and IP6_PINFO usage the style TCP_PINFO and to
some extent IP6_PINFO (and its previous equivalents, sk->tp_pinfo.af_tcp and sk->net_pinfo.ipv6), i.e., using a local variable to hold the result of IP_SK/IP6_PINFO/TCP_PINFO and use this variable instead of the ugly MACRO()->struct_member style. It also fixed a simple error in IP6_PINFO that was causing oopses on IPv6 connections (it was using the tcp area).

The fs unbork patch by Daniel Phillips also uses the same approach
wrt local variables.

It still doesn't make the IPv6 family protocols use each a private
slabcache, i.e., there's still only one slabcache for all IPv6 protocols, I'll work on this RSN.

Patch available at:

http://www.kernel.org/pub/linux/kernel/people/acme/v2.5/2.5.2-pre6
sock.cleanup-2.5.2-pre6.bz2

Comments and test results are welcome.

- Arnaldo