tcp: drop SYN ACK segment for listening sockets

When a SYN ACK is received for a listening socket, just drop it
instead of killing the SYN-cache entry and send a RST.
This closes the possibility to kill a TCP connection during its
handling in the SYN-cache.

Reviewed by: Nick Banks, Peter Lei
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D53540