rhashtable: Extend RCU read lock into rhashtable_insert_rehash()

rhashtable_insert_rehash() requires RCU locks to be held in order
to access ht->tbl and traverse to the last table.

Fixes: ccd57b1bd324 ("rhashtable: Add immediate rehash during insertion")
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/rhashtable.h

@@ -585,8 +585,8 @@ restart:
 	if (unlikely(rht_grow_above_100(ht, tbl))) {
 slow_path:
 		spin_unlock_bh(lock);
-		rcu_read_unlock();
 		err = rhashtable_insert_rehash(ht);
+		rcu_read_unlock();
 		if (err)
 			return err;