bug#19238: Fully fix du circular warning on bind mounts

[Boris Ranto]

The du circular warning can still be hit even though a file system is in
good condition. All we need to do is to get the message is to begin
traversing the file system between the bind mount source and bind mount
target directories, i.e this short script reproduces the problem:

# mkdir -p a/b/c
# mount -o bind a a/b/c
# du a/b

The problem is that in this case, the first directory that is detected
by fts as a duplicate directory is directory a/b/c/b which is not a
mount point.

The solution is to traverse the structure all the way to a/b (excluding
a/b) which is detected as the base of the cycle and look up all these
directories in the mount table.

I'm attaching the patch that fixed this problem for me.

-Boris

0001-du-handle-sub-bind-mount-cycles-gracefully.patch
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