osd/PG: invalidate PG if merging with unexpected version

osd/PG: invalidate PG if merging with unexpected version

If the source or target PG version is 0'0, we may silently take the max
of the source and target and still leave the PG complete.  This
specifically can happen with an empty PG, as seen with bug 38655.  In
theory we could encounter one of the PGs with some other last_update
that doesn't match what we expect.  If that ever happens, make sure the
result is incomplete so that backfill can clean up.

Additionally check that the pool metadata for the last merge matches the
PGs at all.  This could mismatch if we have an osdmap gap and are forced
to do some merge without merge info at all... in which case we should
definitely invalidate: there should be newer copies of the PG(s), and we
have no idea whether the PGs we are merging are what we want.  If this is
some disaster recovery situation, an operator is always free to use
ceph-objectstore-tool to re-mark a PG complete (at their own peril!).

Fixes: http://tracker.ceph.com/issues/38655
Signed-off-by: Sage Weil <sage@redhat.com>