net/packet: annotate data race in packet_sendmsg()

[ Upstream commit d1b5bee4 ]

There is a known race in packet_sendmsg(), addressed
in commit 32d3182c

 ("net/packet: fix race in tpacket_snd()")

Now we have data_race(), we can use it to avoid a future KCSAN warning,
as syzbot loves stressing af_packet sockets :)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>