reiserfs: balance_leaf refactor, pull out balance_leaf_paste_left

This patch factors out a new balance_leaf_paste_left from the code in
balance_leaf responsible for pasting new content into an existing item
located in the node to the left of S[0] in the tree.

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