xfs: lift the XBF_IOEND_FAIL handling into xfs_buf_ioend_disposition

Keep all the error handling code together.

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>