osd: ignore num_objects_dirty for old pools

osd: ignore num_objects_dirty for old pools

Way back in a0ed9c20048750fd4b2c7ce0339fa8b20ef08ca3 we introduced the
dirty flag, but we did not track it in the stats until much later in
c561d5ea22c335e4c05fdc16ca8fb41f75f89d81.  Unfortunately this interval
spans the emperor release.  To avoid making scrub error out and require
repair on *any* of those old pools, flag stats that were encoded before
now such that the dirty stats are ignored.  Clear the flag if we *do*
do a repair so that it will be tracked properly thereafter.

Signed-off-by: Sage Weil <sage@inktank.com>