arm64: cmpxchg: avoid memory barrier on comparison failure

cmpxchg doesn't require memory barrier semantics when the value
comparison fails, so make the barrier conditional on success.

Reviewed-by: Steve Capper <steve.capper@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>