[NETLINK]: Remove references to process ID

People treating the *_pid fields in netlink as a process ID has caused
endless confusion over the years.  The fact that our own netlink.h
does this only adds to the confusion.

So here is a patch to change the comments to refer to it as the port
ID which hopefully will make it clear what the purpose of the fields
really is.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>