osd/PG: do not purge strays in premerge state

osd/PG: do not purge strays in premerge state

The point of premerge is to ensure that the constituent parts of the
target PG are fully clean.  If there is an intervening PG migration and
one of the halves finishes migrating before the other, one half could
get removed and the final merge could result in an incomplete PG.  In the
worst case, the two halves (let's call them A and B) could have started
out together on say [0,1,2], A moves to [3,4,5] and gets deleted from
[0,1,2], and then the final merge happens such that *all* copies of the PG
are incomplete.

We could construct a clever check that does allow removal of strays when
the sibling PG is also ready to go, but it would be complicated.  Do the
simple thing.  In reality, this would be an extremely hard case to hit
because the premerge window is generally very short.

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