MIPS: context switch: Use save/restore instead of set/clear for Status.CU2

Some processors (such as Loongson-3) need to enable CU2 in kernel mode,
current set/clear method will lose Status.CU2 during context switching,
so use save/restore method instead.

Signed-off-by: Huacai Chen <[e-mail]>
Signed-off-by: Thomas Bogendoerfer <[e-mail]>

arch/mips/include/asm/switch_to.h

···
 		__restore_dsp(next);					\
 	}								\
 	if (cop2_present) {						\
+		u32 status = read_c0_status();				\
+									\
 		set_c0_status(ST0_CU2);					\
 		if ((KSTK_STATUS(prev) & ST0_CU2)) {			\
 			if (cop2_lazy_restore)				\
···
 		    !cop2_lazy_restore) {				\
 			cop2_restore(next);				\
 		}							\
-		clear_c0_status(ST0_CU2);				\
+		write_c0_status(status);				\
 	}								\
 	__clear_r5_hw_ll_bit();						\
 	__clear_software_ll_bit();					\