[IPV6]: return EINVAL for invalid address with flowlabel lease request

Currently, when an application requests a lease for a flowlabel via the 
IPV6_FLOWLABEL_MGR socket option, no error is returned if an invalid type 
of destination address is supplied as part of the request, leading to a 
silent failure.  This patch ensures that EINVAL is returned to the 
application in this case.

Signed-off-by: James Morris <jmorris@namei.org>
Signed-off-by: David S. Miller <davem@davemloft.net>