xfs: clear the read/write flags later in xfs_buf_ioend

Clear the flags at the end of xfs_buf_ioend so that they can be used
during the completion.

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>