sunrpc: add netns inum and srcaddr to debugfs rpc_xprt info

The output format should provide a value that matches the one in
the /proc/<pid>/ns/net symlink. This makes it simpler to match the
rpc_xprt and rpc_clnt to a particular container.

Also, when the xprt defines the get_srcaddr operation, use that to
display the source address as well.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>