nullfs: adopt VV_CROSSLOCK

When the lower filesystem directory hierarchy is the same as the nullfs
mount point (admittedly not likely to be a useful situation in
practice), nullfs is subject to the exact deadlock between the busy
count drain and the covered vnode lock that VV_CROSSLOCK is intended
to address.

Reviewed by: kib
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D37458