ipv6: Fix the return interface index when get it while no message is received.

ipv6: Fix the return interface index when get it while no message is received.

When get receiving interface index while no message is received,
the bounded device's index of the socket should be returned.

RFC 3542:
   Issuing getsockopt() for the above options will return the sticky
   option value i.e., the value set with setsockopt().  If no sticky
   option value has been set getsockopt() will return the following
   values:

   -  For the IPV6_PKTINFO option, it will return an in6_pktinfo
      structure with ipi6_addr being in6addr_any and ipi6_ifindex being
      zero.

Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>