tunneling: Add missing rcu_dereference() to cache cleaner.

tunneling: Add missing rcu_dereference() to cache cleaner.

The cleaner for the header caching accesses the tunnel port table
without holding any locks.  However, it doesn't have a read memory
barrier, so there is no guarantee that the contents of the table
have made it to the right CPU.

Found by sparse.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>