arm64: Honour !PTE_WRITE in set_pte_at() for kernel mappings

Currently, set_pte_at() only checks the software PTE_WRITE bit for user
mappings when it sets or clears the hardware PTE_RDONLY accordingly. The
kernel ptes are written directly without any modification, relying
solely on the protection bits in macros like PAGE_KERNEL. However,
modifying kernel pte attributes via pte_wrprotect() would be ignored by
set_pte_at(). Since pte_wrprotect() does not set PTE_RDONLY (it only
clears PTE_WRITE), the new permission is not taken into account.

This patch changes set_pte_at() to adjust the read-only permission for
kernel ptes as well. As a side effect, existing PROT_* definitions used
for kernel ioremap*() need to include PTE_DIRTY | PTE_WRITE.

(additionally, white space fix for PTE_KERNEL_ROX)

Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Reported-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>