rhashtable: fix rht_for_each_entry_safe() endless loop

"next" is not updated, causing an endless loop for buckets with more than
one element.

Fixes: 88d6ed15 ("rhashtable: Convert bucket iterators to take table and index")
Signed-off-by: Patrick McHardy <kaber@trash.net>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>