net/sched: cbs: Fix integer overflow in cbs_set_port_rate()

net/sched: cbs: Fix integer overflow in cbs_set_port_rate()

The subsequent calculation of port_rate = speed * 1000 * BYTES_PER_KBIT,
where the BYTES_PER_KBIT is of type LL, may cause an overflow.
At least when speed = SPEED_20000, the expression to the left of port_rate
will be greater than INT_MAX.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Elena Salomatkina <esalomatkina@ispras.ru>
Link: https://patch.msgid.link/20241013124529.1043-1-esalomatkina@ispras.ru
Signed-off-by: Jakub Kicinski <kuba@kernel.org>