pfkey: fix SA and SP flush sequence

RFC 2367 says flushing behavior should be:
1) user space -> kernel: flush
2) kernel: flush
3) kernel -> user space: flush event to ALL listeners

This is not realistic today in the presence of selinux policies
which may reject the flush etc. So we make the sequence become:
1) user space -> kernel: flush
2) kernel: flush
3) kernel -> user space: flush response to originater from #1
4) if there were no errors then:
kernel -> user space: flush event to ALL listeners

Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>