rgw: write meta of a MP part to a correct pool

rgw: write meta of a MP part to a correct pool

When uploading a multipart part to a non-default storage class,
original logic writes the object data to the correct pool
for its storage class.
But when completing the upload, there's another write op
(with a bunch of setxattrs) to the head object targeting
the standard pool (because non-multipart uploads
write their head objects to the default pool).
As a result of this setxattrs op, a redundant zero-sized object is
created in a wrong pool. Multipart complete, multipart abort and
GC would never try to it clean up.

The fix is to make RGWRados::Object::Write::_do_write_meta() learn the
difference between multipart and non-multipart head objects and pick the
correct placement rule for RGWRados::get_obj_head_ref().

Fixes: http://tracker.ceph.com/issues/49128
Signed-off-by: Jeegn Chen <jeegnchen@tencent.com>