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