Sometime we might want to learn actual OSD utilization by means alternative to regular stats reporting.
This patch enables such an accounting during fsck.
E.g. this could be helpful to investigate stats reporting discrepancies or something.
Relevant log output looks like the following:

... 2 bluestore(dev/osd0) _fsck_on_open Per-pool stats:
... 2 bluestore(dev/osd0) _fsck_on_open pool -1 -> (58 objects, 0 shared blobs, 278 omaps, 6798 bytes in omap keys, 52863 bytes in omap vals, 70038 bytes stored, 229376 bytes allocated)
... 2 bluestore(dev/osd0) _fsck_on_open pool 1 -> (3 objects, 0 shared blobs, 0 omaps, 0 bytes in omap keys, 0 bytes in omap vals, 459280 bytes stored, 462848 bytes allocated)
... 2 bluestore(dev/osd0) _fsck_on_open pool 2 -> (44 objects, 0 shared blobs, 0 omaps, 0 bytes in omap keys, 0 bytes in omap vals, 0 bytes stored, 0 bytes allocated)
... 1 bluestore(dev/osd0) _fsck_on_open <<<FINISH>>> with 0 errors, 0 warnings, 0 repaired, 0 remaining in 0.039815 seconds

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