x86/boot: Adjust .text/.rodata/etc permissions in one place

x86/boot: Adjust .text/.rodata/etc permissions in one place

At the moment, we have two locations selecting restricted permissions, not
very far apart on boot, dependent on opposite answers from using_2M_mapping().
The later location however can shatter superpages if needed, while the former
cannot.

Collect together all the permission adjustments at the slightly later point in
boot, as we likely need to shatter a superpage to support __ro_after_init.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>