MFC r355264: Handle PTB message consistent for SCTP/IPv[46]

Update the hostcache also for PTB messages received for SCTP/IPv6.
The corresponding code for SCTP/IPv4 was introduced in
https://svnweb.freebsd.org/base?view=revision&revision=317597

Submitted by: Julius Flohr
Differential Revision: https://reviews.freebsd.org/D22605