tcp: improve ref count handling when processing SYN

Don't leak a reference count for so->so_cred when processing an
incoming SYN segment with an on-stack syncache entry and the
sysctl variable net.inet.tcp.syncache.see_other is false.

Reviewed by: cc, markj, rscheff
MFC after: 1 week
Sponsored by: Netflix, Inc.
Pull Request: https://reviews.freebsd.org/D46793