powerpc/603: Fix handling of the DIRTY flag

If a page is already mapped RW without the DIRTY flag, the DIRTY
flag is never set and a TLB store miss exception is taken forever.

This is easily reproduced with the following app:

void main(void)
{
	volatile char *ptr = mmap(0, 4096, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);

	*ptr = *ptr;
}

When DIRTY flag is not set, bail out of TLB miss handler and take
a minor page fault which will set the DIRTY flag.

Fixes: f8b58c64 ("powerpc/603: let's handle PAGE_DIRTY directly")
Cc: stable@vger.kernel.org # v5.1+
Reported-by: Doug Crawford <doug.crawford@intelight-its.com>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/80432f71194d7ee75b2f5043ecf1501cf1cca1f3.1566196646.git.christophe.leroy@c-s.fr

arch/powerpc/kernel/head_32.S

@@ -557,9 +557,9 @@ DataStoreTLBMiss:
 	cmplw	0,r1,r3
 	mfspr	r2, SPRN_SPRG_PGDIR
 #ifdef CONFIG_SWAP
-	li	r1, _PAGE_RW | _PAGE_PRESENT | _PAGE_ACCESSED
+	li	r1, _PAGE_RW | _PAGE_DIRTY | _PAGE_PRESENT | _PAGE_ACCESSED
 #else
-	li	r1, _PAGE_RW | _PAGE_PRESENT
+	li	r1, _PAGE_RW | _PAGE_DIRTY | _PAGE_PRESENT
 #endif
 	bge-	112f
 	lis	r2, (swapper_pg_dir - PAGE_OFFSET)@ha	/* if kernel address, use */