Adjusted label names and comments

I changed the label name in syncache_add() to tfo_expanded, as
tfo_socket_created is misleading in the expand failure case (as jtl
pointed out), and tfo_expanded emphasizes that it is only used if we
make it to syncache_tfo_expand() (as opposed to being a more general
purpose destination during TFO processing).

I also changed the new_tfo_socket label in tcp_input() to
tfo_socket_result as sometimes you have a NULL socket at the point it
is used.

The new comments should now be correct.

I updated the comment at the top of syncache_add() to focus on
describing the TFO ACKed-data exception case (the original intent)
without attempting to describe any of the detailed mechanics of when
that path is followed (a distraction and maintenance hazard).

I did try to find a way to truly collapse all the failure-case pending
counter decrements to one location by moving the TFO socket creation
failure decrement out of syncache_tfo_expand(), but I couldn't come up
with an approach that resulted in an overall simplification.