xfs: simplify xfs_ioend_can_merge

Compare the block layer status directly instead of converting it to
an errno first.

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>