iomap: Describe @private in iomap_readahead()

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 <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202601261111.vIL9rhgD-lkp@intel.com/
Fixes: 8806f279244b ("iomap: stash iomap read ctx in the private field of iomap_iter")
Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Link: https://patch.msgid.link/20260213022812.766187-1-lihongbo22@huawei.com
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>