powerpc/8xx: Fix kernel vs user address comparison

Since commit 9132a2e8 ("powerpc/8xx: Define a MODULE area below
kernel text"), module exec space is below PAGE_OFFSET so not only
space above PAGE_OFFSET, but space above TASK_SIZE need to be seen
as kernel space.

Until now the problem went undetected because by default TASK_SIZE
is 0x8000000 which means address space is determined by just
checking upper address bit. But when TASK_SIZE is over 0x80000000,
PAGE_OFFSET is used for comparison, leading to thinking module
addresses are part of user space.

Fix it by using TASK_SIZE instead of PAGE_OFFSET for address
comparison.

Fixes: 9132a2e8 ("powerpc/8xx: Define a MODULE area below kernel text")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/3f574c9845ff0a023b46cb4f38d2c45aecd769bd.1724173828.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/head_8xx.S

@@ -41,12 +41,12 @@
 #include "head_32.h"
 
 .macro compare_to_kernel_boundary scratch, addr
-#if CONFIG_TASK_SIZE <= 0x80000000 && CONFIG_PAGE_OFFSET >= 0x80000000
+#if CONFIG_TASK_SIZE <= 0x80000000 && MODULES_VADDR >= 0x80000000
 /* By simply checking Address >= 0x80000000, we know if its a kernel address */
 	not.	\scratch, \addr
 #else
 	rlwinm	\scratch, \addr, 16, 0xfff8
-	cmpli	cr0, \scratch, PAGE_OFFSET@h
+	cmpli	cr0, \scratch, TASK_SIZE@h
 #endif
 .endm
 
@@ -404,7 +404,7 @@ FixupDAR:/* Entry point for dcbx workaround. */
 	mfspr	r10, SPRN_SRR0
 	mtspr	SPRN_MD_EPN, r10
 	rlwinm	r11, r10, 16, 0xfff8
-	cmpli	cr1, r11, PAGE_OFFSET@h
+	cmpli	cr1, r11, TASK_SIZE@h
 	mfspr	r11, SPRN_M_TWB	/* Get level 1 table */
 	blt+	cr1, 3f