odp-util: Clear padding in the nd_extension.

odp-util: Clear padding in the nd_extension.

Silimar to the patch 67eb8110171f ("odp-util: Fix passing
uninitialized bytes in OVS_KEY_ATTR_CT_ORIG_TUPLE_IPV*.")
when change from flow into the netlink format, the tail
padding of nd_extension should be cleared.

this fixes the following warning logs:

 |ofproto_dpif_upcall(pmd-...)|WARN|Conflicting ukey for flows:
   ufid:763c7d3b-4d0c-4bff-aafc-fdfb6089c2ba
   <...>,eth(...),eth_type(0x86dd),ipv6(...),icmpv6(type=135,code=0),\
   nd(target=fdbd:dc02:ff:1:1::1,sll=fa:16:3e:75:b3:a9,tll=00:00:00:00:00:00),\
   nd_ext(nd_reserved=0x0,nd_options_type=1)

   ufid:763c7d3b-4d0c-4bff-aafc-fdfb6089c2ba
   <...>,eth(...),eth_type(0x86dd),ipv6(...),icmpv6(type=135,code=0),\
   nd(target=fdbd:dc02:ff:1:1::1,sll=fa:16:3e:75:b3:a9,tll=00:00:00:00:00:00),\
   nd_ext(nd_reserved=0x0,nd_options_type=1)
 |ofproto_dpif_upcall(pmd-...)|WARN|upcall_cb failure: ukey installation fails

Fixes: 9b2b84973db7 ("Support for match & set ICMPv6 reserved and options type fields")
Signed-off-by: Peng He <hepeng.0320@bytedance.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>