sh: Fix occasional FPU register corruption under preempt.

Presently with preempt enabled there's the possibility to be preempted
after the TIF_USEDFPU test and the register save, leading to bogus
state post-__switch_to(). Use an explicit preempt_disable()/enable()
pair around unlazy_fpu()/clear_fpu() to avoid this. Follows the x86
change.

Reported-by: Takuo Koguchi <takuo.koguchi.sw@hitachi.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>

arch/sh/kernel/cpu/sh2a/fpu.c

@@ -13,6 +13,7 @@
 #include <linux/signal.h>
 #include <asm/processor.h>
 #include <asm/io.h>
+#include <asm/fpu.h>
 
 /* The PR (precision) bit in the FP Status Register must be clear when
  * an frchg instruction is executed, otherwise the instruction is undefined.