test_rhashtable: don't use global entries variable

pass the entries to test as an argument instead.
Followup patch will add an rhlist test case; rhlist delete opererations
are slow so we need to use a smaller number to test it.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>