use consistent values when consuming runtime-changeable parameters

use consistent values when consuming runtime-changeable parameters

There's no guarantee that e.g. a switch() control expression's memory
operand(s) get(s) read just once. Guard against the compiler producing
"unexpected" code by sprinkling around some ACCESS_ONCE().

I'm leaving alone opt_conswitch[]: It gets accessed in quite a few
places anyway, and an intermediate change won't have any severe effect
afaict.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>