x86/fpu: Update kernel's FPU state before using for the fsave header

In commit

  39388e80 ("x86/fpu: Don't save fxregs for ia32 frames in copy_fpstate_to_sigframe()")

I removed the statement

|       if (ia32_fxstate)
|               copy_fxregs_to_kernel(fpu);

and argued that it was wrongly merged because the content was already
saved in kernel's state.

This was wrong: It is required to write it back because it is only
saved on the user-stack and save_fsave_header() reads it from task's
FPU-state. I missed that part…

Save x87 FPU state unless thread's FPU registers are already up to date.

Fixes: 39388e80 ("x86/fpu: Don't save fxregs for ia32 frames in copy_fpstate_to_sigframe()")
Reported-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Tested-by: Eric Biggers <ebiggers@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: kvm ML <kvm@vger.kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20190607142915.y52mfmgk5lvhll7n@linutronix.de