Btrfs: Avoid delayed reference update looping

btrfs_split_leaf and btrfs_del_items can end up in a loop
where one is constantly spliting a given leaf and the other
is constantly merging it back with the adjacent nodes.

There is a better fix for this, but in the interest of something
small, this patch just changes btrfs_del_items back to balancing less
often.

Signed-off-by: Chris Mason <chris.mason@oracle.com>