Always start IPv6 DAD asynchronously.

Otherwise we transmit the first neighbour solicitation in the context of the
caller of nd6_dad_start(), which can easily result in lock recursion. When
DAD is to be started after some delay, we send the first NS from the DAD
callout handler, so just change the implementation to do this in the
non-delayed case as well.

Reviewed by: ae, hrs
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D6639