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/process_32.c

@@ -25,6 +25,7 @@
 #include <asm/pgalloc.h>
 #include <asm/system.h>
 #include <asm/ubc.h>
+#include <asm/fpu.h>
 
 static int hlt_counter;
 int ubc_usercnt = 0;