sit: fix prefix length of ll and v4mapped addresses

When the local IPv4 endpoint is wilcard (0.0.0.0), the prefix length is
correctly set, ie 64 if the address is a link local one or 96 if the address is
a v4 mapped one.
But when the local endpoint is specified, the prefix length is set to 128 for
both kind of address. This patch fix this wrong prefix length.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>