powerpc/32: Preserve cr1 in exception prolog stack check to fix build error

THREAD_ALIGN_SHIFT = THREAD_SHIFT + 1 = PAGE_SHIFT + 1
Maximum PAGE_SHIFT is 18 for 256k pages so
THREAD_ALIGN_SHIFT is 19 at the maximum.

No need to clobber cr1, it can be preserved when moving r1
into CR when we check stack overflow.

This reduces the number of instructions in Machine Check Exception
prolog and fixes a build failure reported by the kernel test robot
on v5.10 stable when building with RTAS + VMAP_STACK + KVM. That
build failure is due to too many instructions in the prolog hence
not fitting between 0x200 and 0x300. Allthough the problem doesn't
show up in mainline, it is still worth the change.

Fixes: 98bf2d3f ("powerpc/32s: Fix RTAS machine check with VMAP stack")
Cc: stable@vger.kernel.org
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/5ae4d545e3ac58e133d2599e0deb88843cb494fc.1612768623.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/head_32.h

@@ -47,7 +47,7 @@
 	lwz	r1,TASK_STACK-THREAD(r1)
 	addi	r1, r1, THREAD_SIZE - INT_FRAME_SIZE
 1:
-	mtcrf	0x7f, r1
+	mtcrf	0x3f, r1
 	bt	32 - THREAD_ALIGN_SHIFT, stack_overflow
 #else
 	subi	r11, r1, INT_FRAME_SIZE		/* use r1 if kernel */

arch/powerpc/kernel/head_book3s_32.S

@@ -269,12 +269,6 @@ MachineCheck:
 7:	EXCEPTION_PROLOG_2
 	addi	r3,r1,STACK_FRAME_OVERHEAD
 #ifdef CONFIG_PPC_CHRP
-#ifdef CONFIG_VMAP_STACK
-	mfspr	r4, SPRN_SPRG_THREAD
-	tovirt(r4, r4)
-	lwz	r4, RTAS_SP(r4)
-	cmpwi	cr1, r4, 0
-#endif
 	beq	cr1, machine_check_tramp
 	twi	31, 0, 0
 #else