df: bugfix information is not collected when specified FSType

[takahashi-tsc]

ChangeLog: plugin df: Information is not collected when specified FSType

Currently, items ignored by FSType specification are included in duplicate check targets, so some items to be collected are removed by mistake. With this change, only items used for information collection are duplicate check targets.

[takahashi-tsc]

This seems to be a fix for #2465. The same problem also occurred in my environment. I think that without this patch, "collecting all xfs types infromation including /" will be impossible.

And, this patch includes a memory leak bug, but it is fixed by #2599.

[jkohen]

@takahashi-tsc can you give more details on how this fixes the problem? @dago asked for an extra reviewer, and the code itself looks good to me, but I'm struggling to understand what the change in behavior is. I'm clearly missing something, because the before and after look functionally equivalent to me.

[kwiatrox]

For me it is also not clear what is the difference and how it relates to #2465. The new implementation can find also duplicated pointers, but I don't know if that was the reason for this fix. Some more explanation is required as it makes things more complicated and we need justification for that. @takahashi-tsc @dago

[yellowpattern]

This fix is over complicated for what's required - making "ignored" by "df" an attribute of the mounted filesystem list.