libbpf: Remove unnecessary type cast

The link variable is already of type 'struct bpf_link *', casting it to
'struct bpf_link *' is redundant, drop it.

Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220424143420.457082-1-ytcoode@gmail.com