rhashtable: Fix cmpxchg RCU warnings

As cmpxchg is a non-RCU mechanism it will cause sparse warnings
when we use it for RCU.  This patch adds explicit casts to silence
those warnings.  This should probably be moved to RCU itself in
future.

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