powerpc/32s: Avoid crossing page boundary while changing SRR0/1.

Trying VMAP_STACK with KVM, vmlinux was not starting.
This was due to SRR0 and SRR1 clobbered by an ISI due to
the rfi being in a different page than the mtsrr0/1:

c0003fe0 <mmu_off>:
c0003fe0:       38 83 00 54     addi    r4,r3,84
c0003fe4:       7c 60 00 a6     mfmsr   r3
c0003fe8:       70 60 00 30     andi.   r0,r3,48
c0003fec:       4d 82 00 20     beqlr
c0003ff0:       7c 63 00 78     andc    r3,r3,r0
c0003ff4:       7c 9a 03 a6     mtsrr0  r4
c0003ff8:       7c 7b 03 a6     mtsrr1  r3
c0003ffc:       7c 00 04 ac     hwsync
c0004000:       4c 00 00 64     rfi

Align the 4 instruction block used to deactivate MMU to order 4,
so that the block never crosses a page boundary.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/30d2cda111b7977227fff067fa7e358440e2b3a4.1576916812.git.christophe.leroy@c-s.fr

arch/powerpc/kernel/head_32.S

@@ -917,6 +917,8 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_HPTE_TABLE)
 	ori	r4,r4,2f@l
 	tophys(r4,r4)
 	li	r3,MSR_KERNEL & ~(MSR_IR|MSR_DR)
+
+	.align	4
 	mtspr	SPRN_SRR0,r4
 	mtspr	SPRN_SRR1,r3
 	SYNC
@@ -1058,6 +1060,8 @@ _ENTRY(update_bats)
 	rlwinm	r0, r6, 0, ~MSR_RI
 	rlwinm	r0, r0, 0, ~MSR_EE
 	mtmsr	r0
+
+	.align	4
 	mtspr	SPRN_SRR0, r4
 	mtspr	SPRN_SRR1, r3
 	SYNC
@@ -1097,6 +1101,8 @@ mmu_off:
 	andi.	r0,r3,MSR_DR|MSR_IR		/* MMU enabled? */
 	beqlr
 	andc	r3,r3,r0
+
+	.align	4
 	mtspr	SPRN_SRR0,r4
 	mtspr	SPRN_SRR1,r3
 	sync