o In re_newbuf() and re_encap() if re_dma_map_desc() aborts the mapping

o In re_newbuf() and re_encap() if re_dma_map_desc() aborts the mapping
  operation as it ran out of free descriptors or if there are too many
  segments in the first place, call bus_dmamap_unload() in order to
  unload the already loaded segments.
  For trying to map the defragmented mbuf (chain) in re_encap() this
  introduces re_dma_map_desc() setting arg.rl_maxsegs to 0 as a new
  failure mode. Previously we just ignored this case, corrupting our
  view of the TX ring.
o In re_txeof():
  - Don't clear IFF_DRV_OACTIVE unless there are at least 4 free TX
    descriptors. Further down the road re_encap() will bail if there
    aren't at least 4 free TX descriptors, causing re_start() to
    abort and prepend the dequeued mbuf again so it makes no sense
    to pretend we could process mbufs again when in fact we won't.
    While at it replace this magic 4 with a macro RL_TX_DESC_THLD
    throughout this driver.
  - Don't cancel the watchdog timeout as soon as there's at least one
    free TX descriptor but instead only if all descriptors have been
    handled. It's perfectly normal, especially in the DEVICE_POLLING
    case, that re_txeof() is called when only a part of the enqueued
    TX descriptors have been handled, causing the watchdog to be
    disarmed prematurely.
o In re_encap():
  - If m_defrag() fails just drop the packet like other NIC drivers
    do. This should only happen when there's a mbuf shortage, in which
    case it was possible to end up with an IFQ full of packets which
    couldn't be processed as they couldn't be defragmented as they
    were taking up all the mbufs themselves. This includes adjusting
    re_start() to not trying to prepend the mbuf (chain) if re_encap()
    has freed it.
  - Remove dupe initialization of members of struct rl_dmaload_arg to
    values that didn't change since trying to process the fragmented
    mbuf chain.
    While at it remove an unused member from struct rl_dmaload_arg.
o In re_start() remove a abandoned, banal comment. The corresponding
  code was moved to re_attach() some time ago.

With these changes re(4) now survives one day (until stopped) of
hammering out packets here.

Reviewed by:	yongari
MFC after:	2 weeks