nullfs: remove the vhold/vdrop cycle around unlock

Both lower vnode and null data are safe while the lock is held,
at the same time neither is touched after unlock.

While here remove stale comment about interlock handling. It is no
longer legal to pass to unlock.

Reviewed by: kib
Tested by: pho (previous version)
Differential Revision: https://reviews.freebsd.org/D38761