xfs: online repair of realtime summaries

This series uses the atomic extent swapping code to enable safe
reconstruction of the realtime summary file.

First, we enable online repair to allocate a temporary file on the
filesystem that can be used by online repair to stage reconstructed
metadata.  Next, we walk the realtime bitmap to compute a new summary in
this temporary file.  Finally, we swap the contents of the two files
with the atomic extent swap feature.
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