Fix for missing network interface address event when adding the default IPv6

Fix for missing network interface address event when adding the default IPv6
based link-local address.

The default link local address for IPv6 is added as part of bringing the
network interface up. Move the call to "EVENTHANDLER_INVOKE(ifaddr_event,)"
from the SIOCAIFADDR_IN6 ioctl(2) handler to in6_notify_ifa() which should
catch all the cases of adding IPv6 based addresses to a network interface.
Add a witness warning in case the event handler is not allowed to sleep.

Reviewed by:	network (ae), kib
Differential Revision:	https://reviews.freebsd.org/D13407
MFC after:	1 week
Sponsored by:	Mellanox Technologies