BugLink: http://bugs.launchpad.net/bugs/1650607

[ Upstream commit 95c2027b ]

Add a validation function to make sure offset is valid:
1. Not below skb head (could happen when offset is negative).
2. Validate both 'offset' and 'at'.
Signed-off-by: Amir Vadai <amir@vadai.me>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>