mount: fix consistency issue, invalidate mount meta cache more aggressively

[nivekuil]

I upgraded weed mount from 2.49 and it seems there's a consistency problem. Picture's worth a thousand words: this is weed server with 2 mounts on 2 servers. I ran these commands roughly alternating between these two servers. You can see that the hi file seems to be getting clobbered, as if the file length metadata wasn't updating:

With my change it now alternates as expected:

I'm not totally sure why this works, just looked through the commits and this one looked suspicious: nivekuil@e6ba2f9