ieee802154: 6lowpan: trivial checks at first

This patch moves some trivial checks at first before calling
skb_share_check which could do some memcpy if the buffer is shared.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>