osdc/Objecter: fix osd target for newly-homeless op

If we recalculate the mapping and find that there is no primary, we need to
set the 'osd' field to -1.  Otherwise, the caller will try to resend to a
dead session with bad results.

This was introduced in the refactor 860d72770c.

Fixes: #8130 Signed-off-by: Sage Weil <sage@inktank.com>