SUNRPC create an iterator to list only OFFLINE xprts

Create a new iterator helper that will go thru the all the transports
in the switch and return transports that are marked OFFLINE.

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