dp-packet: Enhance packet batch APIs.

dp-packet: Enhance packet batch APIs.

One common use case of 'struct dp_packet_batch' is to process all
packets in the batch in order. Add an iterator for this use case
to simplify the logic of calling sites,

Another common use case is to drop packets in the batch, by reading
all packets, but writing back pointers of fewer packets. Add macros
to support this use case.

Signed-off-by: Andy Zhou <azhou@ovn.org>
Acked-by: Jarno Rajahalme <jarno@ovn.org>