libbpf: Add getter for program size

This adds a new getter for the BPF program size (in bytes). This is useful
for a caller that is trying to predict how much memory will be locked by
loading a BPF object into the kernel.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/157333185272.88376.10996937115395724683.stgit@toke.dk