rework the transmit and receive paths to address reliability issues.

rework the transmit and receive paths to address reliability issues.

phessler@ has been having trouble with msk on overdrive 1000s. some
of the issues relate to the driver not coping with exhaustion of
mbufs for the rx ring, the other issues are corruption of the mcl9k
pool that msk uses.

this diff adds a timeout that the rx refill code uses when the rx
ring is empty and cannot be filled. it'll periodically retry the
ring refill until it can get some mbufs in the air again.

the current code made hunting for the mcl9k issue too hard, so this
rewrites it to be simpler and more like other drivers. there's now
just arrays of mbuf pointers and dmamaps to shadow the hardware
ring entries, and producer and consumer indexes. what was there
before had linkes lists of something to hold mbuf pointers and
dmamaps, and some way to go from the ring to go back to that. i
think, it was hard to tell what was happening.

this also copies the ADDR64 handling on the tx ring to the rx ring.
this potentially makes more rx descriptors available, but that can
happen later.

in hindsight the mcl9k problem could have been from letting if_rxr
allocate the entier ring. if every descriptor was filled, the chip
may have run around the ring when it shouldnt have. giving rxr one
less descriptor than there is on the ring may have fixed the problem
too.

this work also makes it easier to make msk mpsafe.

tested by an ok phessler@
ok kettenis@ deraadt@