ipv6: fix potential "struct net" leak in inet6_rtm_getaddr()

It seems that if userspace provides a correct IFA_TARGET_NETNSID value
but no IFA_ADDRESS and IFA_LOCAL attributes, inet6_rtm_getaddr()
returns -EINVAL with an elevated "struct net" refcount.

Fixes: 6ecf4c37

 ("ipv6: enable IFA_TARGET_NETNSID for RTM_GETADDR")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Ahern <dsahern@kernel.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>