native_network_stack: Fix use after free/missing wait in dhcp
native_network_stack: Fix use after free/missing wait in dhcp

Fixes #184

DHCP code could cause incoming packets to filter with freed dhcp object
if smp calls to unregister did not complete immediately.
In fact, neither register nor de-reg waited for the smp invokes, which
could also make us miss packets -> fail dhcp.

Message-Id: <c68a13be45d3f30e580d47284d97535ae2824fec@scylladb.com>

dreamsxin/seastar,scylladb/seastar,cloudius-systems/seastar,scylladb/seastar,cloudius-systems/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar,cloudius-systems/seastar,syuu1228/seastar,avikivity/seastar,avikivity/seastar,dreamsxin/seastar,syuu1228/seastar,dreamsxin/seastar