datapath: The definition of struct tbl is needed by rcu_dereference()

Due to the use of typeof(*p) by __rcu_dereference_check(),
which was introduced by the Linux changeset
ca5ecddfa8fcbd948c95530e7e817cee9fb43a3d
(rcu: define __rcu address space modifier for sparse) the definition
of the type of a the parameter to rcu_dereference() is needed.

In terms of the datapath, this means that the definition of
struct tbl needs to be available wherever rcu_dereference(dp->table)
is called.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Jesse Gross <jesse@nicira.com>