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 <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz> 
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>