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: NDavid Chinner <dgc@sgi.com>
Signed-off-by: NNathan Scott <nscott@aconex.com>
Signed-off-by: NTim Shimmin <tes@sgi.com>