powerpc/8xx: fix handling of early NULL pointer dereference

NULL pointers are pointers to user memory space. So user pagetable
has to be set in order to avoid random behaviour in case of NULL
pointer dereference, otherwise we may encounter random memory
access hence Machine Check Exception from TLB Miss handlers.

Set user pagetable as early as possible in order to properly
catch early kernel NULL pointer dereference.
Signed-off-by: NChristophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/head_8xx.S

@@ -873,6 +873,10 @@ start_here:
 	li	r0,0
 	stwu	r0,THREAD_SIZE-STACK_FRAME_OVERHEAD(r1)
 
+	lis	r6, swapper_pg_dir@ha
+	tophys(r6,r6)
+	mtspr	SPRN_M_TW, r6
+
 	bl	early_init	/* We have to do this with MMU on */
 
 /*
@@ -893,9 +897,6 @@ start_here:
 	 * init's THREAD like the context switch code does, but this is
 	 * easier......until someone changes init's static structures.
 	 */
-	lis	r6, swapper_pg_dir@ha
-	tophys(r6,r6)
-	mtspr	SPRN_M_TW, r6
 	lis	r4,2f@h
 	ori	r4,r4,2f@l
 	tophys(r4,r4)