ARM: Fix wrong dmb

The __kuser_cmpxchg code uses an ARMv6 dmb instruction, rather than
one based upon the architecture being built for.  Switch to using
the macro provided for this purpose, which also eliminates the
need for an ifdef.

Acked-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>