x86/fpu: Fix i486 + no387 boot crash by only saving FPU registers on context switch if there is an FPU

Booting an i486 with "no387 nofxsr" ends with with the following crash:

   math_emulate: 0060:c101987d
   Kernel panic - not syncing: Math emulation needed in kernel

on the first context switch in user land.

The reason is that copy_fpregs_to_fpstate() tries FNSAVE which does not work
as the FPU is turned off.

This bug was introduced in:

  f1c8cd01 ("x86/fpu: Change fpu->fpregs_active users to fpu->fpstate_active")

Add a check for X86_FEATURE_FPU before trying to save FPU registers (we
have such a check in switch_fpu_finish() already).
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Fixes: f1c8cd01 ("x86/fpu: Change fpu->fpregs_active users to fpu->fpstate_active")
Link: http://lkml.kernel.org/r/20181016202525.29437-4-bigeasy@linutronix.deSigned-off-by: Ingo Molnar <mingo@kernel.org>