crimson/osd: call at_exit() before stopping the engine

crimson/osd: call at_exit() before stopping the engine

if we mkfs and stop the engine, any calls using the engine should/will
fail after `seastar::engine().exit(0)`. in the case of reactor::at_exit(),
it has:

assert(!_stopping);

in this change, we register the at_exit() calls before scheduling the
`engine().exit(0)` call.

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