classify: increase metadata from 16- to 32-bits

classify: increase metadata from 16- to 32-bits

The metadata in classifier entries is used to index a fib or a dpo in
the acl nodes which can exceeds UINT16_MAX in large configurations.
To maintain entries size and alignment, decrease next_index from 32- to
16-bits: next_index should not exceed 16-bits in VPP, as it is already
shown by vlib_buffer_enqueue_to_next() or dpo_id_t.dpoi_next_node.

Type: fix

Change-Id: I4fd1b3cd495319420044c219036b2d2ea952270a
Signed-off-by: Benoît Ganne <bganne@cisco.com>