rgw: data sync: set num_shards when building full maps

rgw: data sync: set num_shards when building full maps

When radosgw-admin data sync init is called on a cluster, the next run
of rgw crashes as when it processes ListBucketIndexesCR, num_shards
isn't set which is later referenced in ListBucketIndexesCR. Setting the
n sync_info.num_shards correctly to handle this case

Fixes: http://tracker.ceph.com/issues/22083
Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>
(cherry picked from commit 4015a57c473d896164b0617be93777d0947a9576)

Conflicts:
	src/rgw/rgw_data_sync.cc