ARM: 6955/1: cmpxchg syscall should data abort if page not write

If the page to cmpxchg is user mode read only (not write),
we should simulate a data abort first.

Signed-off-by: Po-Yu Chuang <ratbert@faraday-tech.com>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>