x86/mtrr: Rename prepare_set() and post_set()

Rename the currently MTRR-specific functions prepare_set() and
post_set() in preparation to move them. Make them non-static and put
their prototypes into cacheinfo.h, where they will end after moving them
to their final position anyway.

Expand the comment before the functions with an introductory line and
rename two related static variables, too.

  [ bp: Massage commit message. ]

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20221102074713.21493-5-jgross@suse.com


Signed-off-by: Borislav Petkov <bp@suse.de>