Support tunable to control Tx deferred packet list limits

Also increase default for Tx queue get-list limit.
Too small limit results in TCP packets drops especiall when many
streams are running simultaneously.
Put list may be kept small enough since it is just a temporary
location if transmit function can't get Tx queue lock.

Submitted by: Andrew Rybchenko <arybchenko at solarflare.com>
Sponsored by: Solarflare Communications, Inc.