syncache_socket(): fix abort path by calling pru_abort directly

syncache_socket(): fix abort path by calling pru_abort directly

On syncache_socket() abort path, the inp's {fport,lport,faddr,laddr}
is either incomplete or half destroyed, which causes tcp_soport()
mapping the socket to a wrong CPU.  However, in syncache code, inp
is only accessed or created by its owner CPU, i.e. we are already on
the inp's owner CPU during the socket aborting.

Add soabort_oncpu(), which calls pru_abort directly.  Use it on
syncache_socket() abort path.