xfrm: release neighbor upon dst destruction

Neighbor is cloned in xfrm6_fill_dst but seems to never be released.
Neighbor entry should be released when XFRM6 dst entry is destroyed
in xfrm6_dst_destroy, otherwise references may be kept forever on
the device pointed by the neighbor entry.

I may not have understood all the subtleties of XFRM & dst so I would
be happy to receive comments on this patch.

Signed-off-by: Romain Kuntz <r.kuntz@ipflavors.com>
Signed-off-by: David S. Miller <davem@davemloft.net>