packet: fix tp_reserve race in packet_set_ring

packet: fix tp_reserve race in packet_set_ring

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: 8913336a7e8d ("packet: add PACKET_RESERVE sockopt")
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>
CVE-2017-1000111

(cherry-picked from commit c27927e372f0785f3303e8fad94b85945e2c97b7)
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>