child-sa: Install drop policies while updating IPsec SAs and policies

child-sa: Install drop policies while updating IPsec SAs and policies

If we have to remove and reinstall SAs for address updates (as with the
Linux kernel) there is a short time where there is no SA installed.  If
we keep the policies installed they (or any traps) might cause acquires
and temporary kernel states that could prevent the updated SA from
getting installed again.

This replaces the previous workaround to avoid plaintext traffic leaks
during policy updates, which used low-priority drop policies.