CVE-2026-53282: x86/kexec: Push kjump return address even for non-kjump kexec

In the Linux kernel, the following vulnerability has been resolved:

x86/kexec: Push kjump return address even for non-kjump kexec

The version of purgatory code shipped by kexec-tools attempts to look above the top of its stack to find a return address for a kjump, even in a non-kjump kexec.

After the commit in Fixes: the word above the stack might not be there, leading to a fault (which is at least now caught by my exception-handling code in kexec).

That commit fixed things for the actual kjump path, but no longer "gratuitously" pushes the unused return address to the stack in the non-kjump path. Put that back in the non-kjump path, to prevent purgatory from crashing when trying to access it.