I/OAT: Add completion callback for async_tx interface use

The async_tx interface includes a completion callback.  This adds support
for using that callback, including using interrupts on completion.

[akpm@linux-foundation.org: various fixes]
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>