sunrpc: provide showing transport's state info in the sysfs directory

In preparation of being able to change the xprt's state, add a way
to show currect state of the transport.

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