core: Copy inner_protocol in copy_skb_header()

inner_protocol was added to struct sk_buff in
0d89d2035fe063461a5ddb609b2c12e7fb006e44 ("MPLS: Add limited GSO support"),
which is scheduled to be included in v3.11.

That patch did not update __copy_skb_header to copy the inner_protocol.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>