x86/fpu: Make the fpu state change in fpu__clear() scheduler-atomic

Do this temporarily only, to make it easier to change the FPU state machine,
in particular this change couples the fpu->fpregs_active and fpu->fpstate_active
states: they are only set/cleared together (as far as the scheduler sees them).

This will be removed by later patches.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Eric Biggers <ebiggers3@gmail.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Yu-cheng Yu <yu-cheng.yu@intel.com>
Link: http://lkml.kernel.org/r/20170923130016.21448-17-mingo@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>