o Split blk_queue_bounce() into a slow and fast path. The fast path is
  inlined, only if we actually need to check the bio for possible
  bounces (and bounce) do we enter __blk_queue_bounce() slow path.

o Fix a nasty bug that could cause corruption for file systems not
  using PAGE_CACHE_SIZE blok size! We were not correctly setting the
  'to' bv_offset correctly.

o Add BIO_BOUNCE flag. Later patches will use this for debug checking.