UPSTREAM: packet: fix tp_reserve race in packet_set_ring

[ Upstream commit c27927e3 ]

Updates to tp_reserve can race with reads of the field in
packet_set_ring. Avoid this by holding the socket lock during
updates in setsockopt PACKET_RESERVE.

This bug was discovered by syzkaller.

Fixes: 8913336a ("packet: add PACKET_RESERVE sockopt")
Change-Id: Ifbceac3542a1b7f25290c3e21008fa76c59fe391
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Marissa Wall <marissaw@google.com>