The soisconnected() call removed from syncache_socket() in r307966 was
not extraneous in the TCP Fast Open (TFO) passive-open case. In the
TFO passive-open case, syncache_socket() is being called during
processing of a TFO SYN bearing a valid cookie, and a call to
soisconnected() is required in order to allow the application to
immediately consume any data delivered in the SYN and to have a chance
to generate response data to accompany the SYN-ACK. The removal of
this call to soisconnected() effectively converted all TFO passive
opens to having the same RTT cost as a standard 3WHS.

This commit adds a call to soisconnected() to syncache_tfo_expand() so
that it is only in the TFO passive-open path, thereby restoring TFO
passve-open RTT performance and preserving the non-TFO connection-rate
performance gains realized by r307966.

MFC after: 1 week
Sponsored by: Limelight Networks