mm: replace pte_write with pte_access_permitted in fault + gup paths

The 'access_permitted' helper is used in the gup-fast path and goes
beyond the simple _PAGE_RW check to also:

 - validate that the mapping is writable from a protection keys
   standpoint

 - validate that the pte has _PAGE_USER set since all fault paths where
   pte_write is must be referencing user-memory.

Link: http://lkml.kernel.org/r/151043111604.2842.8051684481794973100.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: "Jérôme Glisse" <jglisse@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/gup.c

@@ -66,7 +66,7 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
  */
 static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
 {
-	return pte_write(pte) ||
+	return pte_access_permitted(pte, WRITE) ||
 		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
 }