With this the data dependency is reduced to just making sure that the first
member of both struct sock and struct tcp_tw_bucket are a struct sock_common.

Also makes it easier to grep for struct sock and struct tcp_tw_bucket usage in
the tree as all the members in those structs are prefixed, respectively, with
sk_ and tw_, like struct inode (i_), struct block_device (bd_), etc.

Checked namespace with make tags/ctags, just one colision with the macros for
the struct sock members, with a wanrouter struct, fixed that
s/sk_state/state_sk/g in the wanrouter struct.

Checked as well if the names of the members in both structs collided with some
macro, none found.