sunrpc: add xprt id

This adds a unique identifier for a sunrpc transport in sysfs, which is
similarly managed to the unique IDs of clients.

Signed-off-by: Dan Aloni <dan@kernelim.com>
Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>