netdevsim: 'support' multi-buf XDP

Don't error out on large MTU if XDP is multi-buf.
The ping test now tests ping with XDP and high MTU.
netdevsim doesn't actually run the prog (yet?) so
it doesn't matter if the prog was multi-buf..

Reviewed-by: Michal Kubiak <michal.kubiak@intel.com>
Link: https://patch.msgid.link/20250311092820.542148-1-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>