ipv4: Fix traffic triggered IPsec connections.

A recent patch removed the dst_free() on the allocated
dst_entry in ipv4_blackhole_route(). The dst_free() marked the
dst_entry as dead and added it to the gc list. I.e. it was setup
for a one time usage. As a result we may now have a blackhole
route cached at a socket on some IPsec scenarios. This makes the
connection unusable.

Fix this by marking the dst_entry directly at allocation time
as 'dead', so it is used only once.

Fixes: b838d5e1

 ("ipv4: mark DST_NOGC and remove the operation of dst_free()")
Reported-by: Tobias Brunner <tobias@strongswan.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>