netfilter: restore POST_ROUTING hook in NF_HOOK_COND

Commit 2249065f ("netfilter: get rid of the grossness in netfilter.h")
inverted the logic for conditional hook invocation, breaking the
POST_ROUTING hook invoked by ip_output().

Correct the logic and remove an unnecessary initialization.

Reported-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>