x86-64, fpu: Fix %cs value in convert_from_fxsr()

While %ds still contains the userspace selector, %cs is KERNEL_CS at
this point.  Always get %cs from pt_regs even for the current task.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Acked-by: Pekka Enberg <penberg@kernel.org>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
LKML-Reference: <1283563039-3466-7-git-send-email-brgerst@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>