[XFS] Write at EOF may not update filesize correctly.

The recent fix for preventing NULL files from being left around does not
update the file size corectly in all cases. The missing case is a write
extending the file that does not need to allocate a block.

In that case we used a read mapping of the extent which forced the use of
the read I/O completion handler instead of the write I/O completion
handle. Hence the file size was not updated on I/O completion.

SGI-PV: 965068
SGI-Modid: xfs-linux-melb:xfs-kern:28657a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Nathan Scott <nscott@aconex.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>