Modern ARMv7-A/R cores optionally implement below new
hardware feature:

- PXN:
Privileged execute-never(PXN) is a security feature. PXN bit
determines whether the processor can execute software from
the region. This is effective solution against ret2usr attack.
On an implementation that does not include the LPAE, PXN is
optionally supported.

This patch set PXN bit on user page table for preventing
user code execution with privilege mode.
Reviewed-by: NCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: NJungseung Lee <js07.lee@gmail.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>