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: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>