osd: handle pg delete vs merge race

osd: handle pg delete vs merge race

Deletion involves an awkward dance between the pg lock and shard locks,
while the merge prep and tracking is "shard down".  If the delete has
finished its work we may find that a merge has since been prepped.

Unwinding the merge tracking is nontrivial, especially because it might
involved a second PG, possibly even a fabricated placeholder one. Instead,
if we delete and find that a merge is coming, undo our deletion and let
things play out in the future map epoch.

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