homed: fix min_free tracking in manager_rebalance_calculate()

homed: fix min_free tracking in manager_rebalance_calculate()

min_free is supposed to track the minimum free space across all home
directories to scale the next rebalance interval. However, it was
incorrectly assigned h->rebalance_size (the home's current total
allocation) instead of new_free (the remaining allocatable space).

This caused the rebalance interval to be computed from allocation sizes
rather than free space, so a nearly-full home would not trigger the
shorter intervals it should, delaying response to low-space conditions.

Signed-off-by: dongshengyuan <dongshengyuan@uniontech.com>