tipc: factor stats struct out of the larger link struct

tipc: factor stats struct out of the larger link struct

This is done to improve readability, and so that we can give
the struct a name that will allow us to declare a local
pointer to it in code, instead of having to always redirect
through the link struct to get to it.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>