When initializing the TCP FO client cookie cache, take into account
whether the TCP FO support is enabled or not for the client side.

The code in tcp_fastopen_init() implicitly assumed that the sysctl
variable V_tcp_fastopen_client_enable was initialized to 0. This
was initially true, but was changed in r335610, which unmasked this
bug.

Thanks to Pieter de Goeje for reporting the issue on freebsd-net@