x86: make x86_mcinfo_reserve() clear its result buffer

x86: make x86_mcinfo_reserve() clear its result buffer

... instead of all but one of its callers.

Also adjust the corresponding sizeof() expressions to specify the
pointed-to type of the result variable rather than the literal type
(so that a type change of the variable will imply the size to get
adjusted too).

Suggested-by: Ian Campbell <Ian.Campbell@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>