kernel - Add breadcb() function - async I/O w/completion callback

kernel - Add breadcb() function - async I/O w/completion callback

* Add breadcb().  This function performs an asynchronous read for the
  requested buffer and issues the specified callback on completion.
  If the buffer is already B_CACHE the callback is made synchronously
  and BIO_DONE is pre-set on the bio so the callback function can
  determine if I/O occured or not (and whether it must bpdone() the buf).

* This function works differently from and is more flexible than the
  previously removed breada(), and will be used by HAMMER.