powerpc/mm: Fix conditions to perform MMU specific management by blocks on PPC32.

Setting init mem to NX shall depend on sinittext being mapped by
block, not on stext being mapped by block.

Setting text and rodata to RO shall depend on stext being mapped by
block, not on sinittext being mapped by block.

Fixes: 63b2bc61

 ("powerpc/mm/32s: Use BATs for STRICT_KERNEL_RWX")
Cc: stable@vger.kernel.org
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/7d565fb8f51b18a3d98445a830b2f6548cb2da2a.1589866984.git.christophe.leroy@csgroup.eu