cifs: update iface_last_update on each query-and-update

iface_last_update was an unused field when it was introduced.
Later, when we had periodic update of server interface list,
this field was used regularly to decide when to update next.

However, with the new logic of updating the interfaces, it
becomes crucial that this field be updated whenever
parse_server_interfaces runs successfully.

This change updates this field when either the server does
not support query of interfaces; so that we do not query
the interfaces repeatedly. It also updates the field when
the function reaches the end.

Fixes: aa45dadd ("cifs: change iface_list from array to sorted linked list")
Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>