powerpc/603: Clear C bit when PTE is read only

On book3s/32 MMU, PP bits don't offer kernel RO protection,
kernel pages are always RW.

However, on the 603 a page fault is always generated when the
C bit (change bit = dirty bit) is not set.

Enforce kernel RO protection by clearing C bit in TLB miss
handler when the page doesn't have _PAGE_RW flag.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/bbb13848ff0100a76ee9ea95118058c30ae95f2c.1643613343.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/head_book3s_32.S

@@ -507,7 +507,9 @@ DataLoadTLBMiss:
 	/* Convert linux-style PTE to low word of PPC-style PTE */
 	rlwinm	r1,r0,32-9,30,30	/* _PAGE_RW -> PP msb */
 	rlwimi	r0,r0,32-1,30,30	/* _PAGE_USER -> PP msb */
+	rlwimi	r1,r0,32-3,24,24	/* _PAGE_RW -> _PAGE_DIRTY */
 	rlwimi	r0,r0,32-1,31,31	/* _PAGE_USER -> PP lsb */
+	xori	r1,r1,_PAGE_DIRTY	/* clear dirty when not rw */
 	ori	r1,r1,0xe04		/* clear out reserved bits */
 	andc	r1,r0,r1		/* PP = user? rw? 1: 3: 0 */
 BEGIN_FTR_SECTION