Add missing kernel lock around (*if_ioctl)() call within

Add missing kernel lock around (*if_ioctl)() call within
in{,6}_addmulti(). Since kernel lock is no more taken while following
setsockopt() path, it should be taken in this place. Corresponding
in{,6}_delmulti() already acquire kernel lock around (*if_ioctl)().

Problem reported and diff tested by weerd@

ok kn@ bluhm@