udp: lockless UDP_ENCAP_L2TPINUDP / UDP_GRO

[ Upstream commit ac9a7f4c ]

Move udp->encap_enabled to udp->udp_flags.

Add udp_test_and_set_bit() helper to allow lockless
udp_tunnel_encap_enable() implementation.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Stable-dep-of: 70a36f57 ("udp: annotate data-races around udp->encap_type")
Signed-off-by: Sasha Levin <sashal@kernel.org>