Allow explicitly assigned IPv6 loopback address to be used in jails

If a jail has an explicitly assigned IPv6 loopback address then allow it
to be used instead of remapping requests for the loopback adddress to the
first IPv6 address assigned to the jail.

This fixes issues where applications attempt to detect their bound port
where they requested a loopback address, which was available, but instead
the kernel remapped it to the jails first address.

This is the same fix applied to IPv4 fix by: r316313

Also:

• Correct the description of prison_check_ip6_locked to match the code.

MFC after: 2 weeks
Relnotes: Yes
Sponsored by: Multiplay