SUNRPC add xps_nunique_destaddr_xprts to xprt_switch_info in sysfs

In sysfs's xprt_switch_info attribute also display the value of
number of transports with unique destination addresses for this
xprt_switch.

Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>