[netstack] Release network buffers more eagerly

In general, it is not valid to assume that buffers passed to
DeliverNetworkPacket will not be retained by the networking stack. Since
we're always making a copy, we also don't need to hold onto the network
buffer while we're processing the copy.

Test: CQ
Change-Id: I67fe2947b175a7effa9bf0433f28d29e259d4535
1 file changed