/sbin/new-kernel-pkg is required for %post, %preun, and %posttrans scriptlets.

But the only dependency in kernel.spec is
   Requires(pre): /sbin/new-kernel-pkg

Instead, to reflect the use in %post and %preun, it should be
   Requires(post): /sbin/new-kernel-pkg
   Requires(preun): /sbin/new-kernel-pkg

It does not seem to be possible to encode the %posttrans scriptlet dependency correctly, but I would suggest the following hack:
   Requires(postun): /sbin/new-kernel-pkg