Most of xfs_trans_bjoin is duplicated in xfs_trans_get_buf,
xfs_trans_getsb and xfs_trans_read_buf.  Add a new _xfs_trans_bjoin
which can be called by all four functions.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NDave Chinner <david@fromorbit.com>
Signed-off-by: NAlex Elder <aelder@sgi.com>