x86/mm/cpa: Optimize same protection check

When the existing mapping is correct and the new requested page protections
are the same as the existing ones, then further checks can be omitted and the
large page can be preserved. The slow path 4k wise check will not come up with
a different result.

Before:

 1G pages checked:                    2
 1G pages sameprot:                   0
 1G pages preserved:                  0
 2M pages checked:                  540
 2M pages sameprot:                 466
 2M pages preserved:                 47
 4K pages checked:               800709
 4K pages set-checked:             7668

After:

 1G pages checked:                    2
 1G pages sameprot:                   0
 1G pages preserved:                  0
 2M pages checked:                  538
 2M pages sameprot:                 466
 2M pages preserved:                 47
 4K pages checked:               560642
 4K pages set-checked:             7668

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Dave Hansen <dave.hansen@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Bin Yang <bin.yang@intel.com>
Cc: Mark Gross <mark.gross@intel.com>
Link: https://lkml.kernel.org/r/20180917143546.424477581@linutronix.de