bpf: Add the missing BPF_LINK_TYPE invocation for sockmap

In the Linux kernel, the following vulnerability has been resolved: bpf: Add the missing BPF_LINK_TYPE invocation for sockmap There is an out-of-bounds read in bpf_link_show_fdinfo() for the sockmap link fd. Fix it by adding the missing BPF_LINK_TYPE invocation for sockmap link Also add comments for bpf_link_type to prevent missing updates in the future.