net: refactor packet queueing code

The packet queue code is fairly standalone, has some complex details and
easily reusable. It makes sense to split it out on its own. This patch
doesn't contain any functional changes.

Patchworks-ID: 35511
Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>