x86/mm: Add prot_sethuge() helper to abstract out _PAGE_PSE handling

We still have some historic cases of direct fiddling of page
attributes with (dangerous & fragile) type casting and address shifting.

Add the prot_sethuge() helper instead that gets the types right and
doesn't have to transform addresses.

( Also add a debug check to make sure this doesn't get applied
  to _PAGE_BIT_PAT/_PAGE_BIT_PAT_LARGE pages. )

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Dave Hansen <dave.hansen@intel.com>