xdp, xsk: constify read-only arguments of some static inline helpers

xdp, xsk: constify read-only arguments of some static inline helpers

Lots of read-only helpers for &xdp_buff and &xdp_frame, such as getting
the frame length, skb_shared_info etc., don't have their arguments
marked with `const` for no reason. Add the missing annotations to leave
less place for mistakes and more for optimization.

Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://patch.msgid.link/20241203173733.3181246-4-aleksander.lobakin@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>