x86: harden use of calc_ler_msr()

x86: harden use of calc_ler_msr()

Avoid calling the function more than once, thus making sure we won't,
under any unusual circumstances, attempt to enable XEN_LER late (which
can't work, for setup_force_cpu_cap() being __init. In turn this then
allows making the function itself __init, too.

While fiddling with section attributes in this area, also move the two
involved variables to .data.ro_after_init.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>