xfs: allow merging ioends over append boundaries

There is no real problem merging ioends that go beyond i_size into an
ioend that doesn't.  We just need to move the append transaction to the
base ioend.  Also use the opportunity to use a real error code instead
of the magic 1 to cancel the transactions, and write a comment
explaining the scheme.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>