xfs: cleanup data end I/O handlers

Currently we have different end I/O handlers for read vs the different
types of write I/O.  But they are all very similar so we could just
use one with a few conditionals and reduce code size a lot.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>
Signed-off-by: Alex Elder <aelder@sgi.com>