reiserfs: balance_leaf refactor, pull out balance_leaf_new_nodes_insert

This patch factors out a new balance_leaf_new_nodes_insert from the code
in balance_leaf responsible for inserting new items 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>