xfs_attr_leaf: use swap macro in xfs_attr3_leaf_rebalance

xfs_attr_leaf: use swap macro in xfs_attr3_leaf_rebalance

Source kernel commit: 1d5bebbafc73d82e5af003cdd2bf8ee5741cd1df

Make use of the swap macro and remove some unnecessary variables.
This makes the code easier to read and maintain. Also, reduces the
stack usage.

This code was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>