packet: fix tp_reserve race in packet_set_ring

[ Upstream commit c27927e372f0785f3303e8fad94b85945e2c97b7 ]

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.

Change-Id: Id329af9b31d593a720718d2f20bb8715939ff3cd
Fixes: 8913336a ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Git-commit: 63364a50
Git-repo: https://android.googlesource.com/kernel/msm


Signed-off-by: Srinivasa Rao Kuppala <srkupp@codeaurora.org>