s390/bpf: Wrap JIT macro parameter usages in parentheses

This change does not alter JIT behavior; it only makes it possible to
safely invoke JIT macros with complex arguments in the future.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191107113211.90105-1-iii@linux.ibm.com