kernel-netlink: When adding a policy do an update if it already exists

kernel-netlink: When adding a policy do an update if it already exists

This may be the case when SAs are reestablished after a crash of the
IKE daemon.

We could actually always do updates.  The kernel doesn't care, the only
difference is the possible EEXIST if XFRM_MSG_NEWPOLICY is used.  The
advantage of not doing this, though, is that we get a warning in the log
if a policy already exists, as that should usually not be the case.