crimson/.../linked_tree_node: fix pivot_idx assertion in balancing

crimson/.../linked_tree_node: fix pivot_idx assertion in balancing

The pivot_idx is defined in the global index space of the combined
left and right nodes, and since each key may have a different byte
size, a pivot index that happens to be equal to r_size can still
result in a valid rebalancing. Therefore, we only exclude the case
where pivot_idx == l_size, which means no actual rebalancing would
occur.

Fixes: https://tracker.ceph.com/issues/72303
Signed-off-by: Chanyoung Park <chaney.p@kakaoenterprise.com>