Call tcp_notify() directly to shoot down routes, rather than
calling in_pcbnotifyall().

This avoids lock contention on tcbinfo due to in_pcbnotifyall()
holding the tcbinfo write lock while walking all connections.

Reviewed by: rrs, karels
MFC after: 2 weeks
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D7251