selftests/bpf: Fix test_lsm on s390x

Use syscall macros to access the setdomainname() arguments; currently
the code uses gprs[2] instead of orig_gpr2 for the first argument.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-16-iii@linux.ibm.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>