ipv6: stub out even more of addrconf_set_dstaddr if SIT is disabled

There is no point in copying the structure from userspace or looking up
a device if SIT support is not disabled and we'll eventually return
-ENODEV anyway.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>