rack: Update the fast send block on setsockopt(2)

Rack caches TCP/IP header for fast send, so it doesn't call
tcpip_fillheaders(). After certain socket option changes,
namely IPV6_TCLASS, IP_TOS and IP_TTL it needs to update
its fast block to be in sync with the inpcb.

Reviewed by: rrs
Differential Revision: https://reviews.freebsd.org/D32655