6LoWPAN: fix skb_copy call

This patch fixes 2 issues in lowpan_skb_deliver function:
1. Check for return status of skb_copy call;
2. Use skb_copy with proper GFP flag, drop check for non-interrupt
context.

Signed-off-by: Alexander Smirnov <alex.bluesman.smirnov@gmail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>