flow-manager: fix off-by-one in flow_hash row allocation

flow-manager: fix off-by-one in flow_hash row allocation

The current code doesn't cover all rows when more than one flow manager is
used. It leaves a single row between ftd->max and ftd->min of the next
manager orphaned. As an example:

    hash_size=1000
    flowmgr_number=3
    range=333

    instance  ftd->min  ftd->max
    0         0         333
    1         334       666
    2         667       1000

    Rows not covered: 333, 666