xfs: online repair of realtime summaries

We now have all the infrastructure we need to repair file metadata.
We'll begin with the realtime summary file, because it is the least
complex data structure.  To support this we need to add three more
pieces to the temporary file code from the previous patchset --
preallocating space in the temp file, formatting metadata into that
space and writing the blocks to disk, and swapping the fork mappings
atomically.

After that, the actual reconstruction of the realtime summary
information is pretty simple, since we can simply write the incore
copy computed by the rtsummary scrubber to the temporary file, swap the
contents, and reap the old blocks.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
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