x86/mm: Use pte_none() to test for empty PTE

The page table manipulation code seems to have grown a couple of
sites that are looking for empty PTEs.  Just in case one of these
entries got a stray bit set, use pte_none() instead of checking
for a zero pte_val().

The use pte_same() makes me a bit nervous.  If we were doing a
pte_same() check against two cleared entries and one of them had
a stray bit set, it might fail the pte_same() check.  But, I
don't think we ever _do_ pte_same() for cleared entries.  It is
almost entirely used for checking for races in fault-in paths.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave@sr71.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Luis R. Rodriguez <mcgrof@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Toshi Kani <toshi.kani@hp.com>
Cc: dave.hansen@intel.com
Cc: linux-mm@kvack.org
Cc: mhocko@suse.com
Link: http://lkml.kernel.org/r/20160708001915.813703D9@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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