x86/alternatives: Rework information passing into nmi_apply_alternatives()

x86/alternatives: Rework information passing into nmi_apply_alternatives()

nmi_apply_alternatives() is soon going to need to dispatch to multiple
functions, and a force parameter is not a good way of passing information.

Introduce ALT_INSNS and ALT_CALLS to pass in at the top level to select the
operation(s) desired.  They represent what will happen when we've separated
the altcalls out of the general alternative instructions infrastructure,
although in the short term we still need to synthesise the force parameter for
_apply_alternatives().

Move two externs to reduce their scope a little.

No functional change.

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