dio: separate map_bh from dio

dio: separate map_bh from dio

Only a single b_private field in the map_bh buffer head is needed after
the submission path.  Move map_bh separately to avoid storing this
information in the long term slab.

This avoids the weird 104 byte hole in struct dio_submit which also needed
to be memseted early.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>