ikev2: Detect and delete duplicate CHILD_SAs under the same IKE_SA

ikev2: Detect and delete duplicate CHILD_SAs under the same IKE_SA

If both peers initiate the connection simultaneously, this might end up in
duplicate CHILD_SAs. In most situations, these get rejected since we now
reject identical policies having the same reqid in the kernel. However, this
doesn't work in all cases, for example when using static reqids for a
connection.

We defer duplicate detection until SA installation. We don't know how the final
TS looks like before then, and creating multiple CHILD_SAs from a single
configuration is feasible in some configurations.

WIP: Can we somehow avoid the race condition when both peers actually initiate
a colliding exchange?

req1 -->
                                           <-- req2
recv req2 & install SA 2 <--
                                           --> recv req1 & install SA 1
send resp2 -->
                                           <-- send resp1
recv resp1 & replace SA2 with 1 <--
                                           --> recv resp2 & replace SA1 with 2