ipv6: do not erase dst address with flow label destination

This patch is following b579035ff766c9412e2b92abf5cab794bff102b6
	"ipv6: remove old conditions on flow label sharing"

Since there is no reason to restrict a label to a
destination, we should not erase the destination value of a
socket with the value contained in the flow label storage.

This patch allows to really have the same flow label to more
than one destination.

Signed-off-by: Florent Fourcot <florent.fourcot@enst-bretagne.fr>
Reviewed-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>