net: 6lowpan: fix reserved space for single frames

This patch fixes patch add handling to take care tail and headroom for
single 6lowpan frames. We need to be sure we have a skb with the right
head and tailroom for single frames. This patch do it by using
skb_copy_expand() if head and tailroom is not enough allocated by upper
layer.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=195059
Reported-by: David Palma <david.palma@ntnu.no>
Reported-by: Rabi Narayan Sahoo <rabinarayans0828@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alexander Aring <aring@mojatatu.com>
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>