nullfs: reduce areas protected by vnode interlock

Some places only take the interlock to hold the vnode, which was a requiremnt
before they started being manipulated with atomics. Use the newly introduced
vholdnz to bump the count.

Reviewed by: kib
Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21358