libbpf: Use func name when pinning programs with LIBBPF_STRICT_SEC_NAME

We can't use section name anymore because they are not unique
and pinning objects with multiple programs with the same
progtype/secname will fail.

  [0] Closes: https://github.com/libbpf/libbpf/issues/273

Fixes: 33a2c75c

 ("libbpf: add internal pin_name")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20211021214814.1236114-2-sdf@google.com