Fix hostname to be returned in an ICMPv6 NI Reply message defined

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:

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

Differential Revision:	https://reviews.freebsd.org/D20207
MFC after:	1 week