reiserfs: balance_leaf refactor, pull out balance_leaf_new_nodes_paste

This patch factors out a new balance_leaf_new_nodes_insert from the code
in balance_leaf responsible for pasting new content into existing items
that may have been shifted into new nodes in the tree.

It has not been reformatted yet.
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Jan Kara <jack@suse.cz>