MFC r259038, r259039:

  Bump the maximum VM space from 3 * memory size to a fixed
  256MB. That's all we have room for since we map the hardware registers
  starting at 0xd0000000. This allows my 64MB AT91SAM9G20 to boot again
  after the unmmaped I/O changes were MFC'd at r251897. Other
  subplatforms may need similar treatment.

  Although not strictly required to boot a 64MB board, bump
  vm_max_virtual_address to be KERNVIRTADDR + 256MB. This allows some
  future shock protection since the KVA requirements have gone up since
  the unmapped changes have gone in, as well as preventing us from
  overlapping with the hardware devices, which we map at 0xd0000000,
  which we'd hit with anything more than 85MB...
MFC r259212, r259220:

  Fix one race and one fence post error. When the TX buffer was
  completely full, we'd not complete any of the mbufs due to the fence
  post error (this creates a large leak). When this is fixed, we still
  leak, but at a much smaller rate due to a race between ateintr and
  atestart_locked as well as an asymmetry where atestart_locked is
  called from elsewhere.  Ensure that we free in-flight packets that
  have completed there as well. Also remove needless check for NULL on
  mb, checked earlier in the loop and simplify a redundant if.