bpf: Expose bpf_d_path helper to sleepable LSM hooks

Sleepable hooks are never called from an NMI/interrupt context, so it
is safe to use the bpf_d_path helper in LSM programs attaching to these
hooks.

The helper is not restricted to sleepable programs and merely uses the
list of sleepable hooks as the initial subset of LSM hooks where it can
be used.

Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20201113005930.541956-3-kpsingh@chromium.org