x86: MSR: add a struct representation of an MSR

Add a struct representing a 64bit MSR pair consisting of a low and high
register part and convert msr_info to use it. Also, rename msr-on-cpu.c
to msr.c.

Side note: Put the cpumask.h include in __KERNEL__ space thus fixing an
allmodconfig build failure in the headers_check target.

CC: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>

arch/x86/lib/Makefile

@@ -2,7 +2,7 @@
 # Makefile for x86 specific library files.
 #
 
-obj-$(CONFIG_SMP) := msr-on-cpu.o
+obj-$(CONFIG_SMP) := msr.o
 
 lib-y := delay.o
 lib-y += thunk_$(BITS).o