When calculating po->tp_hdrlen + po->tp_reserve the result can overflow.

Fix by checking that tp_reserve <= INT_MAX on assign.
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 commit bcc5364b linux-net)
Signed-off-by: Andy Whitcroft <apw@canonical.com>