reiserfs: balance_leaf refactor pull out balance_leaf_finish_node_insert

This patch factors out a new balance_leaf_finish_node_insert from the code
in balance_leaf responsible for inserting new items into S[0]

It has not been reformatted yet.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Jan Kara <jack@suse.cz>