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: ccd57b1b ("rhashtable: Add immediate rehash during insertion")
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>