tcp: Don't try to disconnect a socket multiple times.

When the checks for INP_TIMEWAIT were removed, tcp_usr_close() and
tcp_usr_disconnect() were no longer prevented from calling
tcp_disconnect() on a socket that was already disconnected. This
triggered a panic in cxgbe(4) for TOE where the tcp_disconnect() on an
already-disconnected socket invoked tcp_output() on a socket that was
already in time-wait.

Reviewed by: rrs, np
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D37112