rgw: fix cloud tier multipart resume starting at part number 0

rgw: fix cloud tier multipart resume starting at part number 0

When resuming a cloud tier multipart upload, the part-size
calculation was inside the fresh-init block and never executed.
cur_part, num_parts, and part_size stayed at 0, causing the
remote endpoint to reject part number 0 as invalid.

Move the part-size calculation out of the init block so it
runs for both fresh and resumed uploads.

Signed-off-by: Matthew N. Heler <matthew.heler@hotmail.com>