vfs: implement usecount implying holdcnt

vnodes have 2 reference counts - holdcnt to keep the vnode itself from getting
freed and usecount to denote it is actively used.

Previously all operations bumping usecount would also bump holdcnt, which is
not necessary. We can detect if usecount is already > 1 (in which case holdcnt
is also > 1) and utilize it to avoid bumping holdcnt on our own. This saves
on atomic ops.

Reviewed by: kib
Tested by: pho (previous version)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21471