x86/mtrr: Add comment for set_mtrr_state() serialization

Add a comment about set_mtrr_state() needing serialization.

  [ bp: Touchups. ]

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220820092533.29420-2-jgross@suse.com