net: caif: use skb helpers instead of open-coding them

Use existing skb_put_data() and skb_trim() instead of open-coding them,
with the skb_put_data() first so that logically, `skb` still contains the
data to be copied in its data..tail area when skb_put_data() reads it.
This change on its own is a cleanup, and it is also necessary for potential
future integration of skbuffs with things like KASAN.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>