Various bus_dma(9) fixes.

Various bus_dma(9) fixes.
 - The hardware does not support DAC so limit DMA address space to
   4GB.
 - Removed BUS_DMA_ALLOC_NOW flag.
 - Created separated Tx buffer and Rx buffer DMA tags. Previously
   it used to single DMA tag and it was not possible to specify
   different DMA restrictions.
 - Apply 4 bytes alignment limitation of Tx buffer.
 - Apply 8 bytes alignment limitation of Rx buffer.
 - Tx side bus_dmamap_load_mbuf_sg(9) support.
 - Preallocate Tx DMA maps as creating DMA maps take very long time
   on architectures that require real DMA maps.
 - Adjust guard buffer size to 1522 + 8 as it should include VLAN
   and additional reserved bytes in Rx buffer.
 - Plug memory leak in device detach. Previously wrong buffer
   address was used to free allocated memory.
 - Added rl_list_rx_init() to clear Rx buffer and cleared the
   buffer.
 - Don't destroy DMA maps in rl_txeof() as the DMA map should be
   reused. There is no reason to destroy/recreate the DMA maps in
   this driver.
 - Removed rl_dma_map_rxbuf()/rl_dma_map_txbuf() callbacks.
 - The hardware does not support descriptor based DMA on Tx side
   and the Tx buffer address should be aligned on 4 bytes boundary
   as well as manual padding for short frames. Because of this
   hardware limitation rl(4) always used to invoke m_defrag(9) to
   get a 4 bytes aligned single buffer. However m_defrag(9) takes
   a lot of CPU cycles on slow machines and not all packets need
   the help of m_defrag(9). Armed with the information, don't
   invoke m_defrag(9) if the following conditions are true.
      1. Buffer is not fragmented.
      2. Buffer is aligned on 4 bytes boundary.
      3. Manual padding is not necessary.
      4. Or padding is necessary but upper stack passed a writable
         buffer and the space needed for padding is satisfied.
   This change combined with preallocated DMA maps greatly
   increased Tx performance of driver on sparc64.
 - Moved bus_dmamap_sync(9) in rl_start_locked() to rl_encap() and
   corrected memory synchronization operation specifier of
   bus_dmamap_sync(9).
 - Removed bus_dmamap_unload(9) in rl_stop(). There is no need to
   reload/unload Rx buffer as rl(4) always have to copy from the
   buffer. It just needs proper bus_dmamap_sync(9) calls before
   copying the received frame.

With this change rl(4) should work on systems with more than 4GB
memory.

PR:	kern/128143