xfrm: don't check the default policy if the policy allows the packet

The current code doesn't let a simple "allow" policy counteract a
default policy blocking all incoming packets:

    ip x p setdefault in block
    ip x p a src 192.168.2.1/32 dst 192.168.2.2/32 dir in action allow

At this stage, we have an allow policy (with or without transforms)
for this packet. It doesn't matter what the default policy says, since
the policy we looked up lets the packet through. The case of a
blocking policy is already handled separately, so we can remove this
check.

Fixes: 2d151d39 ("xfrm: Add possibility to set the default to block if we have no policy")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>