rhashtable: silence RCU warning in rhashtable_test.

print_ht in rhashtable_test calls rht_dereference() with neither
RCU protection or the mutex.  This triggers an RCU warning.
So take the mutex to silence the warning.

Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>