osd: fix bad is_active() assert in share_map()

We may be is_stopping() here if we are racing with shutdown().

Fixes: #8465
Backport: firefly, dumpling
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1882 from ceph/wip-8465

osd: fix bad is_active() assert in share_map()

Reviewed-by: Samuel Just <sam.just@inktank.com>