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.