libxfs: factor common xfs_trans_bjoin code

libxfs: factor common xfs_trans_bjoin code

Most of xfs_trans_bjoin is duplicated in xfs_trans_get_buf_map,
xfs_trans_getsb and xfs_trans_read_buf_map.  Add a new
_xfs_trans_bjoin which can be called by all three functions.

Source kernel commit: d7e84f413726876c0ec66bbf90770f69841f7663

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Alex Elder <aelder@sgi.com>
[sandeen: merge to userspace]
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>