ARM: 7314/1: kuser: consistently use usr_ret for returning from helpers

__kuser_cmpxchg64 has a return path using bx lr to get back to the caller.
This is actually ok since the code in question is predicated on
CONFIG_CPU_32v6K, but for the sake of consistency using the usr_ret
macro is probably better.
Acked-by: NDave Martin <dave.martin@linaro.org>
Acked-by: NNicolas Pitre <nico@linaro.org>
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/entry-armv.S

@@ -790,7 +790,7 @@ __kuser_cmpxchg64:				@ 0xffff0f60
 	smp_dmb	arm
 	rsbs	r0, r3, #0			@ set returned val and C flag
 	ldmfd	sp!, {r4, r5, r6, r7}
-	bx	lr
+	usr_ret	lr
 
 #elif !defined(CONFIG_SMP)