powerpc/32: Use SPRN_SPRG_SCRATCH2 in exception prologs

Use SPRN_SPRG_SCRATCH2 as a third scratch register in
exception prologs in order to simplify them and avoid
data going back and forth from/to CR.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/6f5c8a7faa8cc54acb89c55c20aa579a2f30a4e9.1606285014.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/head_32.h

@@ -40,7 +40,7 @@
 
 .macro EXCEPTION_PROLOG_1 for_rtas=0
 #ifdef CONFIG_VMAP_STACK
-	mr	r11, r1
+	mtspr	SPRN_SPRG_SCRATCH2,r1
 	subi	r1, r1, INT_FRAME_SIZE		/* use r1 if kernel */
 	beq	1f
 	mfspr	r1,SPRN_SPRG_THREAD
@@ -61,15 +61,10 @@
 
 .macro EXCEPTION_PROLOG_2 handle_dar_dsisr=0
 #ifdef CONFIG_VMAP_STACK
-	mtcr	r10
-	li	r10, MSR_KERNEL & ~(MSR_IR | MSR_RI) /* can take DTLB miss */
-	mtmsr	r10
+	li	r11, MSR_KERNEL & ~(MSR_IR | MSR_RI) /* can take DTLB miss */
+	mtmsr	r11
 	isync
-#else
-	stw	r10,_CCR(r11)		/* save registers */
-#endif
-	mfspr	r10, SPRN_SPRG_SCRATCH0
-#ifdef CONFIG_VMAP_STACK
+	mfspr	r11, SPRN_SPRG_SCRATCH2
 	stw	r11,GPR1(r1)
 	stw	r11,0(r1)
 	mr	r11, r1
@@ -78,14 +73,12 @@
 	stw	r1,0(r11)
 	tovirt(r1, r11)		/* set new kernel sp */
 #endif
+	stw	r10,_CCR(r11)		/* save registers */
 	stw	r12,GPR12(r11)
 	stw	r9,GPR9(r11)
-	stw	r10,GPR10(r11)
-#ifdef CONFIG_VMAP_STACK
-	mfcr	r10
-	stw	r10, _CCR(r11)
-#endif
+	mfspr	r10,SPRN_SPRG_SCRATCH0
 	mfspr	r12,SPRN_SPRG_SCRATCH1
+	stw	r10,GPR10(r11)
 	stw	r12,GPR11(r11)
 	mflr	r10
 	stw	r10,_LINK(r11)
@@ -99,7 +92,6 @@
 	stw	r10, _DSISR(r11)
 	.endif
 	lwz	r9, SRR1(r12)
-	andi.	r10, r9, MSR_PR
 	lwz	r12, SRR0(r12)
 #else
 	mfspr	r12,SPRN_SRR0