selftests/bpf: fix perf_buffer on s390

perf_buffer test fails for exactly the same reason test_attach_probe
used to fail: different nanosleep syscall kprobe name.

Reuse the test_attach_probe fix.

Fixes: ee5cf82c ("selftests/bpf: test perf buffer API")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>