os/bluestore: _do_remove: dirty shard individually as each blob is unshared

os/bluestore: _do_remove: dirty shard individually as each blob is unshared

Two problems with old code:

1- dirty_shard range is inclusive, so we might dirty the shard after b_end
2- we might unshare blobs in two shards with an unloaded shard in between,
which would mean dirtying a shard that isn't loaded.

Fix by ensuring the shard for each unshared blob is dirty individually.

Fixes: http://tracker.ceph.com/issues/20849
Signed-off-by: Sage Weil <sage@redhat.com>