[XFS] Use the right offset when ensuring a delayed allocate conversion has...

[XFS] Use the right offset when ensuring a delayed allocate conversion has covered the offset originally requested.  Can cause data corruption when multiple processes are performing writeout on different areas of the same file.  Quite difficult to hit though.

SGI Modid: xfs-linux:xfs-kern:22377a

Signed-off-by: Nathan Scott <nathans@sgi.com>
Signed-off-by: Christoph Hellwig <hch@sgi.com>
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