net/packet: fix overflow in check for priv area size

net/packet: fix overflow in check for priv area size

Subtracting tp_sizeof_priv from tp_block_size and casting to int
to check whether one is less then the other doesn't always work
(both of them are unsigned ints).

Compare them as is instead.

Also cast tp_sizeof_priv to u64 before using BLK_PLUS_PRIV, as
it can overflow inside BLK_PLUS_PRIV otherwise.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
BugLink: https://bugs.launchpad.net/bugs/1678009
CVE-2017-7308
(cherry picked from linux-next commit 2b6867c2ce76c596676bec7d2d525af525fdc6e2)
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>