vfs: allow v_holdcnt to transition 0->1 without the interlock

Since r356672 ("vfs: rework vnode list management") there is nothing to do
apart from altering freevnodes count, but this much can be safely done based
on the result of atomic_fetchadd.

Reviewed by: kib
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D23186