With PIE enabled, the swapper PTEs would have a Permission Indirection Index
(PIIndex) of 0. A PIIndex of 0 is not currently used by any other PTEs.

To avoid using index 0 specifically for the swapper PTEs, mark them as
PTE_UXN and PTE_WRITE, so that they map to a PAGE_KERNEL_EXEC equivalent.

This also adds PTE_WRITE to KPTI_NG_PTE_FLAGS, which was tested by booting
with kpti=on.
Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20230606145859.697944-12-joey.gouly@arm.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>