MFC from r347887:

Fix hostname to be returned in an ICMPv6 NI Reply message defined
in RFC 4620, ICMPv6 Node Information Queries. A vnet jail with an
IPv6 address sent a hostname of the host environment, not the
jail, even if another hostname was set to the jail.

This change can be tested by the following commands:

1. ifconfig epair0 create
2. jail -c -n j1 vnet host.hostname=vnetjail path=/ persist
3. ifconfig epair0b vnet j1
4. ifconfig epair0a inet6 -ifdisabled auto_linklocal up
5. jexec j1 ifconfig epair0b inet6 -ifdisabled auto_linklocal up
6. ping6 -w ff02::1%epair0a

Differential Revision: https://reviews.freebsd.org/D20207