arm64/mm: Add pte_rdonly() helper

This replaces open coding PTE_RDONLY check with a new helper pte_rdonly().
No functional change is intended here.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20230713092004.693749-1-anshuman.khandual@arm.com
Signed-off-by: Will Deacon <will@kernel.org>