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.