rapidio/rionet: fix multicast packet transmit logic

Fix multicast packet transmit logic to account for repetitive transmission
of single skb:
- correct check for available buffers (this bug may produce NULL pointer
  crash dump in case of heavy traffic);
- update skb user count (incorrect user counter causes a warning dump from
  net_tx_action routine during multicast transfers in systems with three or
  more rionet participants).

Signed-off-by: Alexandre Bounine <alexandre.bounine@idt.com>
Cc: Matt Porter <mporter@kernel.crashing.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>