Add a new method to the transport switch API to provide a way to convert
the opaque contents of xprt->addr to a human-readable string.

Test plan:
Compile kernel with CONFIG_NFS enabled.
Signed-off-by: NChuck Lever <chuck.lever@oracle.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>