MFC r254008 Make the fallback from MSIX to MSI interrupt usage more graceful.
MFC r254262  Further improve the msix setup, make sure pci_alloc_msix() gives us
             the vectors we requested, and fall back to MSI when not, also release
             any allocated resources before the fallback.
MFC r254264  Make the igb_mq_start() routine use TRYLOCK rather than simple
             queuing, there were latency and other problems with the latter.