x86: fix (and simplify) MTRR overlap checking

x86: fix (and simplify) MTRR overlap checking

Obtaining one individual range per variable range register (via
get_mtrr_range()) was bogus from the beginning, as these registers may
cover multiple disjoint ranges. Do away with that, in favor of simply
comparing masked addresses.

Also, for is_var_mtrr_overlapped()'s result to be correct when called
from mtrr_wrmsr(), generic_set_mtrr() must update saved state first.

As minor cleanup changes, constify is_var_mtrr_overlapped()'s parameter
and make mtrr_wrmsr() static.

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