Make Linux AFS able to work with union mounts

[geofft]

This is a followup to #463. Although we don't intend to use union mounts in AFS in our current design — just in tmpfs and ext[234] — there are definite use cases where it can be useful, such as the thing we've talked about for a while where you can union-mount a locker at /, between the local disk and the login tmpfs, to provide software in more normal locations than /mit/something. Although most of the code is in the VFS layer, each filesystem needs a tiny bit of explicit support for union mounts, among other things the current design wants  additional file types for whiteouts and fallthrus akin to those for fifos, devices, normal files, etc. That support is going into more normal filesystems (including tmpfs and ext); it would be great to have those patches prepared and ready for OpenAFS once union mount support is usable on normal filesystems.

If someone ends up tackling #248, this would be a good thing to look into while you're there.