x86: Serialize EFI time accesses on rtc_lock

x86: Serialize EFI time accesses on rtc_lock

The EFI specification requires that callers of the time related
runtime functions serialize with other CMOS accesses in the
kernel, as the EFI time functions may choose to also use the
legacy CMOS RTC.

Besides fixing a latent bug, this is a prerequisite to safely
enable the rtc-efi driver for x86, which ought to be preferred
over rtc-cmos on all EFI platforms.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Acked-by: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: <mjg@redhat.com>
Link: http://lkml.kernel.org/r/4E257E33020000780004E319@nat28.tlf.novell.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Matthew Garrett <mjg@redhat.com>