Fixes SockMap.value_size so that sockmaps can be dumped by bpftool

[murrayh]

bpftool reports a cryptic "No space left on device" error when attempting to dump a SockMap

This is because the value_size is 4B -- it needs to be 8B to work with bpftool.

With this change, the commands look like:

[murrayh]

Thank-you I have force pushed the the Sign off message