direct-io: move aio_complete into ->end_io

direct-io: move aio_complete into ->end_io

Filesystems with unwritten extent support must not complete an AIO request
until the transaction to convert the extent has been commited.  That means
the aio_complete calls needs to be moved into the ->end_io callback so
that the filesystem can control when to call it exactly.

This makes a bit of a mess out of dio_complete and the ->end_io callback
prototype even more complicated.

Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Signed-off-by: Alex Elder <[email protected]>