Move the fill and the tail pointer change to a later point, to make sure

Move the fill and the tail pointer change to a later point, to make sure
that the RDT is written, and that it is written not too early.  Doing it
before writing IGC_RXDCTL definitely doesn't work.

The tail pointer needs to be set to the next empty slot, so it has to be
"last desc filled + 1".

Make sure sure that the rss mapping does not happen in the middle of the
RX checksum block, and that it happens only if nqueues > 1.  Also disable
storing bad packets.

With this, igc(4) receives packets just fine.

ok kevlo@