selftests/bpf: Add test for d_path helper

Adding test for d_path helper which is pretty much
copied from Wenbo Zhang's test for bpf_get_fd_path,
which never made it in.

The test is doing fstat/close on several fd types,
and verifies we got the d_path helper working on
kernel probes for vfs_getattr/filp_close functions.

Original-patch-by: Wenbo Zhang <ethercflow@gmail.com>

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200825192124.710397-14-jolsa@kernel.org