xdp: introduce xdp_get_shared_info_from_{buff, frame} utility routines

Introduce xdp_get_shared_info_from_{buff,frame} utility routines to get
skb_shared_info from xdp buffer/frame pointer.
xdp_get_shared_info_from_{buff,frame} will be used to implement xdp
multi-buffer support

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>