os/bluestore: fix clone dirty_range again

os/bluestore: fix clone dirty_range again

If we are cloning a blob for a 1 byte logical extent then dirty_range_begin
will equal _end and we won't dirty the source onode (with possibly newly
shared blobs).

Fix by using a separate flag to indicate whether we are dirtying instead
of overloading the begin/end markers for this.  Note that even if they
are equal dirty_range will still dirty the shard in question.

This is a result of 0ae5d92d42500e5ab08253a00bda47b957767ebc.

Fixes: http://tracker.ceph.com/issues/20983
Signed-off-by: Sage Weil <sage@redhat.com>