fix io_uring error

[JacksonYao287]

from the debug info below , we know the io_uring error is caused by the unalignment of offset

$3 = (iomgr::drive_iocb) {
  _vptr.drive_iocb = 0x55555a4a5908 <vtable for iomgr::drive_iocb+16>,
  static inlined_iov_count = 4,
  iodev = 0x55555ae71e00,
  iface = 0x55555acd7720,
  op_type = iomgr::DriveOpType::WRITE,
  size = 1024,
  offset = 10647755115,
  unique_id = 0,
  iovcnt = 1,
  result = -22,
  completion = std::variant<> [no contained value],
  resubmit_cnt = 0,
  part_read_resubmit_cnt = 0,
  initiating_reactor = 0x0,
  iocb_id = 2,
  op_start_time = {
    __d = {
      __r = 22743123547526470
    }
  },
  op_submit_time = {
    __d = {
      __r = 0
    }
  },
  user_data = std::variant<std::array<iovec, 4>, std::unique_ptr<iovec[]>, char *> [index 0] = {{
      _M_elems = {{
          iov_base = 0x55555c3e2400,
          iov_len = 1024
        }, {
          iov_base = 0x0,
          iov_len = 0
        }, {
          iov_base = 0x0,
          iov_len = 0
        }, {
          iov_base = 0x0,
          iov_len = 0
        }}
    }}
}

[JacksonYao287]

we should first compute the final vparam.num_chunks, then compute vparam.vdev_size according to the final vparam.num_chunks, so that we can get a aligned chunk size.

otherwise , we will first get a vdev_size from the num_chunks of direct input vparam.num_chunks, which might may be different from the final vparam.num_chunks and lead to unalignment

for the testcase failure eBay/HomeObject#167
if the input num_chunks is less than the max_number_chunks(with out this PR is 960), then everything goes well. otherwise, since the final vparam.num_chunks is max_number_chunks, but the chunk size is computed according to the input vparam.num_chunks(65000), this will lead to a unaligned chunk size , which will lead to io_uring failuer

[JacksonYao287]

closed , and changes in this PR will be made in #381