tcp: Disallow connecting a disconnected socket

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().

Reported by: syzbot+f0f7871ec5397602b446@syzkaller.appspotmail.com
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40579