reiserfs: balance_leaf refactor, pull out balance_leaf_insert_right

This patch factors out a new balance_leaf_insert_right from the code in
balance_leaf responsible for inserting new items into the node to
the right of S[0] in the tree.

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