VFS changes

Hi,

Here's a bunch a VFS interface changes that are needed to implement
some FUSE features.

The biggest part is patches 1-3, which pass the open file to the
filesystem for syscalls passed an file descriptor, such as fstat(),
fchmod(), etc...  These patches touch a lot of filesystems, but
otherwise quite simple and actually source and binary backward
compatible.

Christoph, can you please take a quick look at the patches, if the
interface changes are OK?

Thanks,
Miklos
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