net: xdp: handle frags with unreadable memory

net: xdp: handle frags with unreadable memory

We don't expect frags with unreadable memory to be presented
to XDP programs today, but the XDP helpers are designed to be
usable whether XDP is enabled or not. Support handling frags
with unreadable memory.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20250905221539.2930285-3-kuba@kernel.org
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>