bonding: Fix negative jump label count on nested bonding

With nested bonding devices the nested bond device's ndo_bpf was
called without a program causing it to decrement the static key
without a prior increment leading to negative count.

Fix the issue by 1) only calling slave's ndo_bpf when there's a
program to be loaded and 2) only decrement the count when a program
is unloaded.

Fixes: 9e2ee5c7 ("net, bonding: Add XDP support to the bonding driver")
Reported-by:  <syzbot+30622fb04ddd72a4d167@syzkaller.appspotmail.com>
Signed-off-by: Jussi Maki <joamaki@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>