tracing: Introduce helpers to safely handle dynamic-sized sockaddrs

Enable a struct sockaddr to be stored in a trace record as a
dynamically-sized field. The common cases are AF_INET and AF_INET6
which are different sizes, and are vastly smaller than a struct
sockaddr_storage.

These are safer because, when used properly, the size of the
sockaddr destination field in each trace record is now guaranteed
to be the same as the source address that is being copied into it.

Link: https://lore.kernel.org/all/164182978641.8391.8277203495236105391.stgit@bazille.1015granger.net/
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>