Allow vdrop() of a vnode not yet on the per-mount list after r306512.

The old code allowed calling vdrop() before insmntque() to place the vnode back
onto the freelist for later recycling. Some downstream consumers may rely on
this support. Normally insmntque() failing is fine since is uses vgone() and
immediately frees the vnode rather than attempting to add it to the freelist if
vdrop() were used instead.

Also assert that vhold() cannot be used on such a vnode.

Reviewed by: kib, cem, markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D12126