rhashtable: Allow other tasks to be scheduled in large lookup loops

Depending on system speed, the large lookup/insert/delete loops of the testsuite can
take a considerable amount of time to complete causing watchdog warnings to appear.
Allow other tasks to be scheduled throughout the loops.

Reported-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>