igb: Introduce multiple TX queues with infrastructure

This code adds multiple Tx queue infrastructure much like we
previously did in ixgbe. The MSI-X vector mapping is the bulk of
the change.

IAM can now be safely enabled and we've verified that it does
work correctly. We can also eliminate the tx ring lock.

Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>