x86/mm: Introduce pte_accessible()

We need pte_present to return true for _PAGE_PROTNONE pages, to indicate that
the pte is associated with a page.

However, for TLB flushing purposes, we would like to know whether the pte
points to an actually accessible page.  This allows us to skip remote TLB
flushes for pages that are not actually accessible.

Fill in this method for x86 and provide a safe (but slower) method
on other architectures.

Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Fixed-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-66p11te4uj23gevgh4j987ip@git.kernel.org
[ Added Linus's review fixes. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>