rhashtable: Fix RCU dereference annotation in rht_bucket_nested

The current annotation is wrong as it says that we're only called
under spinlock.  In fact it should be marked as under either
spinlock or RCU read lock.

Fixes: da20420f ("rhashtable: Add nested tables")
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>