osd/PrimaryLogPG: _delete_oid - fix incorrect 'legacy' flag

osd/PrimaryLogPG: _delete_oid - fix incorrect 'legacy' flag

For pre-Luminous created objects, we shall default 'legacy' flag
to true, so we can still create a snapdir object properly if necessary
for Luminous backward compatibility.

Note that this patch is not going to land on master first
(and hence can not be cherry-picked from master) because it will
finally be deprecated by https://github.com/ceph/ceph/pull/17579,
in which we are going to kill the snapdir object completely for Mimic.

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>