iomap: Describe @Private in iomap_readahead()

The kernel test rebot reports the kernel-doc warning:

```
Warning: fs/iomap/buffered-io.c:624 function parameter 'private'
 not described in 'iomap_readahead'
```

The former commit in "iomap: stash iomap read ctx in the private
field of iomap_iter" has added a new parameter @Private to
iomap_readahead(), so let's describe the parameter.

Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Fixes: 8806f27 ("iomap: stash iomap read ctx in the private field of iomap_iter")
Signed-off-by: Hongbo Li <[email protected]>
Link: https://patch.msgid.link/[email protected]
Reviewed-by: Gao Xiang <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: Hongbo Li

fs/iomap/buffered-io.c

@@ -625,6 +625,7 @@ static int iomap_readahead_iter(struct iomap_iter *iter,
  * iomap_readahead - Attempt to read pages from a file.
  * @ops: The operations vector for the filesystem.
  * @ctx: The ctx used for issuing readahead.
+ * @private: The filesystem-specific information for issuing iomap_iter.
  *
  * This function is for filesystems to call to implement their readahead
  * address_space operation.