bonding: fix off-by-one error

Fix off-by-one error because IFNAMSIZ == 16 and when this
code gets executed we stick a NULL byte where we should not.

How to reproduce:
 with CONFIG_CC_STACKPROTECTOR=y (otherwise it may pass by silently)
 modprobe bonding; echo 1 > /sys/class/net/bond0/bonding/mode;
 echo "AAAAAAAAAAAAAAAA" > /sys/class/net/bond0/bonding/primary;

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>