selftests/bpf: Add path_fd-based BPF_OBJ_PIN and BPF_OBJ_GET tests

Add a selftest demonstrating using detach-mounted BPF FS using new mount
APIs, and pinning and getting BPF map using such mount. This
demonstrates how something like container manager could setup BPF FS,
pin and adjust all the necessary objects in it, all before exposing BPF
FS to a particular mount namespace.

Also add a few subtests validating all meaningful combinations of
path_fd and pathname. We use mounted /sys/fs/bpf location for these.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230523170013.728457-5-andrii@kernel.org