socket: Reorder socket and sockbuf fields to eliminate some padding

[markj]

Summary

This is in preparation for moving sockbuf locks into the owning socket,
in order to provide proper interlocking for listen(2). The saved space
ensures that UMA can provide the same efficiency as before. No
functional change intended.

[tuexen]

You might want to gallatin@ into the loop. He might have an opinion performance wise...