net/packet: convert po->auxdata to an atomic flag

[ Upstream commit fd53c297aa7b077ae98a3d3d2d3aa278a1686ba6 ]

po->auxdata can be read while another thread
is changing its value, potentially raising KCSAN splat.

Convert it to PACKET_SOCK_AUXDATA flag.

Fixes: 8dc41944 ("[PACKET]: Add optional checksum computation for recvmsg")
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>