xfs: use swap macro in xfs_dir2_leafn_rebalance

Make use of the swap macro and remove unnecessary variable *tmp*. This
makes the code easier to read and maintain. Also, slightly refactor some
code.

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>