crimson/osd/main: always stop osd as long as it started

crimson/osd/main: always stop osd as long as it started

otherwise the sharded_service's dtor complains if we destruct it without
stopping it first, like:

FATAL: startup failed: std::system_error (error crimson::net:3, negotiation failure)
crimson-osd: ../src/seastar/include/seastar/core/sharded.hh:523: seastar::sharded<T>::~sharded() [with Service = crimson::osd::OSD]: Assertion `_instances.empty()' failed.
Aborting on shard 0.

Signed-off-by: Kefu Chai <kchai@redhat.com>