reiserfs: balance_leaf refactor, pull out balance_leaf_finish_node_paste

This patch factors out a new balance_leaf_finish_node_paste from the code
in balance_leaf responsible for pasting new content into existing items
held in S[0].

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