udp: move udp->no_check6_rx to udp->udp_flags

udp: move udp->no_check6_rx to udp->udp_flags

[ Upstream commit bcbc1b1de884647aa0318bf74eb7f293d72a1e40 ]

syzbot reported that udp->no_check6_rx can be read locklessly.
Use one atomic bit from udp->udp_flags.

Fixes: 1c19448c9ba6 ("net: Make enabling of zero UDP6 csums more restrictive")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>