Don't define skb_copy_{to,from}_linear_data_offset if it is available.

Don't define skb_copy_{to,from}_linear_data_offset if it is available.

Linux 2.6.22 introduced functions skb_copy_from_linear_data_offset()
and skb_copy_to_linear_data_offset().  In earlier versions we defined them.
But Xen backports these functions, so this became a duplicate definition.
So check for them at configure time instead of depending on the kernel
version number.