osd: Add hit_set_flushing to track current flushes and prevent races

When flushing a HitSet track in hit_set_flushing map so that
agent_load_hit_sets() doesn't try to read it too soon.

Fixes: #7575

Signed-off-by: David Zafman <david.zafman@inktank.com>
Merge pull request #1405 from ceph/wip-7575

osd: Add hit_set_flushing to track current flushes and prevent races

Reviewed-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>