Assign IPv6 link-local address to loopback interfaces whith unit > 0.

When an interface has IFF_LOOPBACK flag in6_ifattach() tries to assing
IPv6 loopback address to this interface. It uses in6ifa_ifpwithaddr()
to check, that interface doesn't already have given address and then
uses in6_ifattach_loopback(). If in6_ifattach_loopback() fails, it just
exits and thus skips assignment of IPv6 LLA.
Fix this using in6ifa_ifwithaddr() function. If IPv6 loopback address is
already assigned in the system, do not call in6_ifattach_loopback().

PR:		[[ https://bugs.freebsd.org/138678 | 138678 ]]