BACKPORT: Smack: IPv6 casting error fix for 3.11

BACKPORT: Smack: IPv6 casting error fix for 3.11

The original implementation of the Smack IPv6 port based
local controls works most of the time using a sockaddr as
a temporary variable, but not always as it overflows in
some circumstances. The correct data is a sockaddr_in6.
A struct sockaddr isn't as large as a struct sockaddr_in6.
There would need to be casting one way or the other. This
patch gets it the right way.

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>
(cherry-picked from upstream 6ea062475a9a2ea6e1394487fa0e51b3459957d1)