lib/odp-execute: Use dpif_packet_set_dp_hash() instead of ->dp_hash

When building with DPDK support, 'struct dpif_packet' won't have 'dp_hash'
member. dpif_packet_set_dp_hash() and dpif_packet_get_dp_hash() should be used.

Furthermore, the masked set action shouldn't read 'md->dp_hash' (which is
shared in a batch), but should use dpif_packet_get_dp_hash() to get each packet
private hash.

This commit fixes the build with DPDK.

Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>