MFC r328540:

MFC r328540:
  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:		138678