We got a little mixed up with what the backoff member holds in the
IPoIB multicast group structure: sometimes it was used as a number of
seconds, and sometimes it was used as a number of jiffies.  Fix the
code so that backoff is always in seconds.
Signed-off-by: Hal Rosenstock <halr@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>