rhashtable: Drop raw RCU deref in nested_table_free

This patch replaces some unnecessary uses of rcu_dereference_raw
in the rhashtable code with rcu_dereference_protected.

The top-level nested table entry is only marked as RCU because it
shares the same type as the tree entries underneath it.  So it
doesn't need any RCU protection.

We also don't need RCU protection when we're freeing a nested RCU
table because by this stage we've long passed a memory barrier
when anyone could change the nested table.

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