sunrpc: display xprt's queuelen of assigned tasks via sysfs

Once a task grabs a trasnport it's reflected in the queuelen of
the rpc_xprt structure. Add display of that value in the xprt's
info file in sysfs.

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