x86/mm/cpa: Better use CLFLUSHOPT

Currently we issue an MFENCE before and after flushing a range. This
means that if we flush a bunch of single page ranges -- like with the
cpa array, we issue a whole bunch of superfluous MFENCEs.

Reorgainze the code a little to avoid this.

[ mingo: capitalize instructions, tweak changelog and comments. ]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tom.StDenis@amd.com
Cc: dave.hansen@intel.com
Link: http://lkml.kernel.org/r/20181203171043.626999883@infradead.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>