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>