powerpc/32s: Fix napping restore in data storage interrupt (DSI)

When a DSI (Data Storage Interrupt) is taken while in NAP mode,
r11 doesn't survive the call to power_save_ppc32_restore().

So use r1 instead of r11 as they both contain the virtual stack
pointer at that point.

Fixes: 4c0104a8 ("powerpc/32: Dismantle EXC_XFER_STD/LITE/TEMPLATE")
Cc: stable@vger.kernel.org # v5.13+
Reported-by: Finn Thain <fthain@linux-m68k.org>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/731694e0885271f6ee9ffc179eb4bcee78313682.1628003562.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/head_book3s_32.S

@@ -300,7 +300,7 @@ ALT_MMU_FTR_SECTION_END_IFSET(MMU_FTR_HPTE_TABLE)
 	EXCEPTION_PROLOG_1
 	EXCEPTION_PROLOG_2 INTERRUPT_DATA_STORAGE DataAccess handle_dar_dsisr=1
 	prepare_transfer_to_handler
-	lwz	r5, _DSISR(r11)
+	lwz	r5, _DSISR(r1)
 	andis.	r0, r5, DSISR_DABRMATCH@h
 	bne-	1f
 	bl	do_page_fault