can: kvaser_pciefd: refine error prone echo_skb_max handling logic

In the Linux kernel, the following vulnerability has been resolved:

can: kvaser_pciefd: refine error prone echo_skb_max handling logic

echo_skb_max should define the supported upper limit of echo_skb[]
allocated inside the netdevice's priv. The corresponding size value
provided by this driver to alloc_candev() is KVASER_PCIEFD_CAN_TX_MAX_COUNT
which is 17.

But later echo_skb_max is rounded up to the nearest power of two (for the
max case, that would be 32) and the tx/ack indices calculated further
during tx/rx may exceed the upper array boundary. Kasan reported this for
the ack case inside kvaser_pciefd_handle_ack_packet(), though the xmit
function has actually caught the same thing earlier.

BUG: KASAN: slab-out-of-bounds in kvaser_pciefd_handle_ack_packet+0x2d7/0x92a drivers/net/can/kvaser_pciefd.c:1528
Read of size 8 at addr ffff888105e4f078 by task swapper/4/0

CPU: 4 UID: 0 PID: 0 Comm: swapper/4 Not tainted 6.15.0 #12 PREEMPT(voluntary)
Call Trace:
<IRQ>
dump_stack_lvl lib/dump_stack.c:122
print_report mm/kasan/report.c:521
kasan_report mm/kasan/report.c:634
kvaser_pciefd_handle_ack_packet drivers/net/can/kvaser_pciefd.c:1528
kvaser_pciefd_read_packet drivers/net/can/kvaser_pciefd.c:1605
kvaser_pciefd_read_buffer drivers/net/can/kvaser_pciefd.c:1656
kvaser_pciefd_receive_irq drivers/net/can/kvaser_pciefd.c:1684
kvaser_pciefd_irq_handler drivers/net/can/kvaser_pciefd.c:1733
__handle_irq_event_percpu kernel/irq/handle.c:158
handle_irq_event kernel/irq/handle.c:210
handle_edge_irq kernel/irq/chip.c:833
__common_interrupt arch/x86/kernel/irq.c:296
common_interrupt arch/x86/kernel/irq.c:286
</IRQ>

Tx max count definitely matters for kvaser_pciefd_tx_avail(), but for seq
numbers' generation that's not the case - we're free to calculate them as
would be more convenient, not taking tx max count into account. The only
downside is that the size of echo_skb[] should correspond to the max seq
number (not tx max count), so in some situations a bit more memory would
be consumed than could be.

Thus make the size of the underlying echo_skb[] sufficient for the rounded
max tx value.

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