x86: fix 32-bit case of __get_user_asm_u64()

The code to fetch a 64-bit value from user space was entirely buggered,
and has been since the code was merged in early 2016 in commit
b2f680380ddf ("x86/mm/32: Add support for 64-bit __get_user() on 32-bit
kernels").

Happily the buggered routine is almost certainly entirely unused, since
the normal way to access user space memory is just with the non-inlined
"get_user()", and the inlined version didn't even historically exist.

The normal "get_user()" case is handled by external hand-written asm in
arch/x86/lib/getuser.S that doesn't have either of these issues.

There were two independent bugs in __get_user_asm_u64():

- it still did the STAC/CLAC user space access marking, even though
that is now done by the wrapper macros, see commit 11f1a4b9755f
("x86: reorganize SMAP handling in user space accesses").

This didn't result in a semantic error, it just means that the
inlined optimized version was hugely less efficient than the
allegedly slower standard version, since the CLAC/STAC overhead is
quite high on modern Intel CPU's.

- the double register %eax/%edx was marked as an output, but the %eax
part of it was touched early in the asm, and could thus clobber other
inputs to the asm that gcc didn't expect it to touch.

In particular, that meant that the generated code could look like
this:

mov (%eax),%eax
mov 0x4(%eax),%edx

where the load of %edx obviously was _supposed_ to be from the 32-bit
word that followed the source of %eax, but because %eax was
overwritten by the first instruction, the source of %edx was
basically random garbage.

The fixes are trivial: remove the extraneous STAC/CLAC entries, and mark
the 64-bit output as early-clobber to let gcc know that no inputs should
alias with the output register.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Benjamin LaHaise <bcrl@kvack.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: stable@kernel.org # v4.8+
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>