Currently nothing prevents tcp_usr_connect() from attempting to connect
when the socket has been disconnected. At the moment, doing so triggers
an assertion in in_pcbconnect() because inp_faddr is not unspecified. I
believe this may have been caught in the past by TIMEWAIT checks, but
those are now removed.

Check for additional socket states in tcp_connect(). Add a regression
test for this case.

Reported by: syzbot+f0f7871ec5397602b446@syzkaller.appspotmail.com