cifs: do not let cifs_chan_update_iface deallocate channels

cifs_chan_update_iface is meant to check and update the server
interface used for a channel when the existing server interface
is no longer available.

So far, this handler had the code to remove an interface entry
even if a new candidate interface is not available. Allowing
this leads to several corner cases to handle.

This change makes the logic much simpler by not deallocating
the current channel interface entry if a new interface is not
found to replace it with.

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>