Around a generic call to null_nodeget(), there is nothing that would
prevent the unmount of the nullfs mp until we process to the
insmntque1() point. Calculate the VV_ROOT flag after insmntque1() to
not access mp->mnt_data before we have an exclusively locked vnode
from this mount point on the mp vnode list.
Reported and tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week