osd: cancel pg merge if PGs are undersized

osd: cancel pg merge if PGs are undersized

If the PG is undersized, cancel the PG merge attempt early.  Undersized is
a bad thing because it makes merge more dangerous.

It's also bad because the PG won't be fully clean when it finishes
peering, which means last_epoch_clean can be something far in the past,
and past_intervals won't be empty.  Since we also take the past_intervals
from the source PG, we want to be confident that it is valid.  It *should*
match up with the target PG since they should have mapped to the same
OSDs since they were both clean at the ReadyToMerge point--in fact, they
should both be empty.  If a PG mapping change snuck in such that they did
map somewhere else, though, the same set of mapping changes will have
applied to both the source and target, so it should be safe.

(It would be better of the mon rejected the ReadyToMerge if the
mapping with the latest OSDMap has changed since the message was sent.
If we do that the situation is even better, but this change is still
appropriate.)

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