dio: use a slab cache for struct dio

dio: use a slab cache for struct dio

A direct slab call is slightly faster than kmalloc and can be better
cached per CPU.  It also avoids rounding to the next kmalloc slab.

In addition this enforces cache line alignment for struct dio to avoid any
false sharing.

Signed-off-by: Andi Kleen <ak@linux.intel.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>