xfs: don't allocate an ioend for direct I/O completions

Back in the days when the direct I/O ->end_io callback could be called
from interrupt context for AIO we needed a structure to hand off to the
workqueue, and reused the ioend structure for this purpose.  These days
->end_io is always called from user or workqueue context, which allows us
to avoid this memory allocation and simplify the code significantly.

[dchinner: removed now unused xfs_finish_ioend_sync() function after
	   Brian Foster did an initial review. ]

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>