mount: fix mnt_resort_siblings to work as described

We should add new entry _before_ first entry with less depth to sort in
descending order.

e.g: entries in list have depths [7,5,3], adding new entry m with depth
4 we would break list_for_each_entry loop on p with depth 3, before
patch we would get [7,5,3,4] after list_add, which is wrong.

Also we can relax "<=" check to "<" to avoid unnecessary reordering.

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>