caif: use pskb_put() instead of reimplementing its functionality

Also remove the warning for fragmented packets -- skb_cow_data() will
linearize the buffer, removing all fragments.

Signed-off-by: Mathias Krause <mathias.krause@secunet.com>
Cc: Dmitry Tarnyagin <dmitry.tarnyagin@lockless.no>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>