x86/kexec: Mark relocate_kernel page as ROX instead of RWX

x86/kexec: Mark relocate_kernel page as ROX instead of RWX

All writes to the page now happen before it gets marked as executable
(or after it's already switched to the identmap page tables where it's
OK to be RWX).

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20241205153343.3275139-14-dwmw2@infradead.org