use xfs_trans_ijoin in xfs_trans_iget

Use xfs_trans_ijoin in xfs_trans_iget in case we need to join an inode into
a transaction instead of opencoding it.  Based on a discussion with and an
incomplete patch from Niv Sardi.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Niv Sardi <xaiki@sgi.com>