unlink, rmdir: call notify_upper from VOP pre method instead of syscall

[kib]

Summary

Suppose that there are two or more nullfs mounts over some fs, and
suppose the we unlink a file on one of the nullfs mount.
This way notify_upper get called from the lower vnode as well, allowing
the other nullfs mounts to note that and drop their caches for the
unlinked vnode.

PR:     254210