x86/fpu: Don't store PKRU in xstate in fpu_reset_fpstate()

x86/fpu: Don't store PKRU in xstate in fpu_reset_fpstate()

PKRU for a task is stored in task->thread.pkru when the task is scheduled
out. For 'current' the authoritative source of PKRU is the hardware.

fpu_reset_fpstate() has two callers:

  1) fpu__clear_user_states() for !FPU systems. For those PKRU is irrelevant

  2) fpu_flush_thread() which is invoked from flush_thread(). flush_thread()
     resets the hardware to the kernel restrictive default value.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121456.802850233@linutronix.de