osd: osdmap: do not assert target_v when encoding under stretch mode

osd: osdmap: do not assert target_v when encoding under stretch mode

As in dd63a3e21fb6d63b8951a6542fb09f98b79b8cd6 for the OSDMap, this
caused crashes when encoding for kernel clients, and is unnecessary
for servers because they are separately gated.

I did a full audit of every instance of "assert" I added to the codebase
to make sure this is the very last one of these issues.

Signed-off-by: Greg Farnum <gfarnum@redhat.com>