SUNRPC enable back offline transports in trunking discovery

When we are adding a transport to a xprt_switch that's already on
the list but has been marked OFFLINE, then make the state ONLINE
since it's been tested now.

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