bpf, docs: Rename bpf_lsm.rst to prog_lsm.rst

This allows for documentation relating to BPF Program Types to be
matched by the glob pattern prog_* for inclusion in a sphinx toctree

Signed-off-by: Dave Tucker <dave@dtucker.co.uk>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: KP Singh <kpsingh@kernel.org>
Link: https://lore.kernel.org/bpf/49fe0f370a2b28500c1b60f1fdb6fb7ec90de28a.1636749493.git.dave@dtucker.co.uk