Fix source-specific routing in Bird 3

Fix source-specific routing in Bird 3

Commit 69d1ffde4c72 ("Split route data structure to storage (ro) /
manipulation (rw) structures.") changed rte->net from a pointer to a
'struct network' to a 'struct net_addr', but kept the address-of (&)
operator before casting to 'net_addr_ip6_sadr *' when sending a
source-specific route to the kernel.

Because 'struct network' had an embedded struct member (struct
fib_node), the address-of was needed to get back to a pointer to the
data, but with the change in the commit mentioned above, e->net is now a
straight pointer to the address.

The bug meant that the source prefixes passed to the kernel were
essentially garbage, leading to routes in the kernel like:

default from b74:9e05:0:1:d8cf:c000::/86 via fe80::1 dev eth0 proto bird metric 32 pref medium

Fix this by getting rid of the address-of operator.

Note by commiter: used our TYPE_CAST macro instead of plain typecast
to avoid this kind of problem in future.

Fixes: 69d1ffde4c72 ("Split route data structure to storage (ro) / manipulation (rw) structures.")
Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Maria Matejka <mq@jmq.cz>