rhashtable: Document the right function parameters

rhashtable_lookup_get_insert_key doesn't have a parameter `data`. It
does have a parameter `key`, however.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>