x86/fpu: Improve xstate_fault() handling

x86/fpu: Improve xstate_fault() handling

There are two problems with xstate_fault handling:

 - The xstate_fault() macro takes an argument, but that's
   propagated into the assembly named label as well. This
   is technically correct currently but might result in
   failures if anytime a more complex argument is used.
   So use a separate '_err' name instead for the label.

 - All the xstate_fault() using functions have an error
   variable named 'err', which is an output variable to
   the asm() they are using. The problem is, it's not always
   set by the asm(), in which case the compiler might
   optimize out its initialization, so that the C variable
   'err' might become corrupted after the asm() - confusing
   anyone who tries to take advantage of this variable
   after the asm(). Mark it an input variable as well.

   This is a latent bug currently, but an upcoming debug
   patch will make use of 'err'.

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>