xsk: handle NULL dereference of the skb without frags issue

xsk: handle NULL dereference of the skb without frags issue

When a first descriptor (xs->skb == NULL) triggers -EOVERFLOW in
xsk_build_skb_zerocopy() (e.g., MAX_SKB_FRAGS exceeded), the
free_err -EOVERFLOW handler unconditionally dereferences xs->skb
via xsk_inc_num_desc(xs->skb) and xsk_drop_skb(xs->skb), causing
a NULL pointer dereference.

Fix this by guarding the existing xsk_inc_num_desc()/xsk_drop_skb()
calls with an xs->skb check (for the continuation case), and add
an else branch for the first-descriptor case that manually cancels
the one reserved CQ slot and increments invalid_descs by one to
account for the single invalid descriptor.

Fixes: cf24f5a5feea ("xsk: add support for AF_XDP multi-buffer on Tx path")
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://patch.msgid.link/20260502200722.53960-4-kerneljasonxing@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>