MFC r340257

MFC r340257

in6_ifattach_linklocal: handle immediate removal of the new LLA

If another thread immediately removes the link-local address
added by in6_update_ifa(), in6ifa_ifpforlinklocal() can return NULL,
so the following assertion (or dereference) is wrong.
Remove the assertion, and handle NULL somewhat better than panicking.
This matches all of the other callers of in6_update_ifa().

PR:		219250
Reviewed by:	bz, dab (both an earlier version)
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D17898