It is not safe to modify the vnode structure after releasing one's
reference. Modify vput() to avoid this. Use refcount_release_if_last()
to opportunistically call vput_final() with the vnode lock held since we
need the vnode lock in order to deactivate the vnode, and it's silly to
drop the vnode lock and immediately reacquire it in this common case.
Note that vunref() has a similar flaw.
Reported by: syzbot+6676b3ff282d590b0fb3@syzkaller.appspotmail.com
Reported by: syzbot+38e26cf6f959e886f110@syzkaller.appspotmail.com