net: fix buffer overflow in af_xdp_umem_create()

net: fix buffer overflow in af_xdp_umem_create()

s->pool has n_descs elements so maximum i should be
n_descs - 1. Fix the upper bound.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: cb039ef3d9 ("net: add initial support for AF_XDP network backend")
Cc: qemu-stable@nongnu.org
Reviewed-by: Ilya Maximets <i.maximets@ovn.org>
Signed-off-by: Anastasia Belova <nabelova31@gmail.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>