Merge branch 'x86-uaccess-cleanup': x86 uaccess header cleanups

Merge my x86 uaccess updates branch.

The LAM ("Linear Address Masking") updates in this release made me
unhappy about how "access_ok()" was done, and it actually turned out to
have a couple of small bugs in it too.  This is my cleanup of the code:

 - use the sign bit of the __user pointer rather than masking the
   address and checking it against the TASK_SIZE range.

   We already did this part for the get/put_user() side, but
   'access_ok()' did the naïve "mask and range check" thing, which not
   only generates nasty code, but also ended up meaning that __access_ok
   itself didn't do a good job, and so copy_from_user_nmi() didn't get
   the check right.

 - move all the code that is 64-bit only into the 64-bit version of the
   header file, so that we don't unnecessarily pollute the shared x86
   code and make it look like LAM might work in 32-bit too.

 - fix a bug in the address masking (that doesn't end up mattering: in
   this case the fix was to just remove the buggy code entirely).

 - a couple of trivial cleanups and added commentary about the
   access_ok() rules.

* x86-uaccess-cleanup:
  x86-64: mm: clarify the 'positive addresses' user address rules
  x86: mm: remove 'sign' games from LAM untagged_addr*() macros
  x86: uaccess: move 32-bit and 64-bit parts into proper <asm/uaccess_N.h> header
  x86: mm: remove architecture-specific 'access_ok()' define
  x86-64: make access_ok() independent of LAM