libublk: fix max cmd buffer size calculation on 64K page size

Signed-off-by: Ming Lei <[email protected]>

src/io.rs

@@ -506,15 +506,13 @@ impl UblkQueue<'_> {
         let depth = dev.dev_info.queue_depth as u32;
         let cdev_fd = dev.cdev_file.as_raw_fd();
         let cmd_buf_sz = UblkQueue::cmd_buf_sz(depth) as usize;
+        let max_cmd_buf_sz = UblkQueue::cmd_buf_sz(sys::UBLK_MAX_QUEUE_DEPTH) as libc::off_t;
 
         ring.submitter()
             .register_files(&tgt.fds[0..tgt.nr_fds as usize])?;
 
-        let off = sys::UBLKSRV_CMD_BUF_OFFSET as libc::off_t
-            + q_id as libc::off_t
-                * ((sys::UBLK_MAX_QUEUE_DEPTH as usize
-                    * core::mem::size_of::<sys::ublksrv_io_desc>())
-                    as libc::off_t);
+        let off =
+            sys::UBLKSRV_CMD_BUF_OFFSET as libc::off_t + (q_id as libc::off_t) * max_cmd_buf_sz;
         let io_cmd_buf = unsafe {
             libc::mmap(
                 std::ptr::null_mut::<libc::c_void>(),