ovl: add splice file read write helper

#include <linux/xattr.h>

#include <linux/uio.h>

#include <linux/uaccess.h>

#include <linux/splice.h>

#include <linux/mm.h>

#include <linux/fs.h>

#include "overlayfs.h"

struct ovl_aio_req {

	return ret;

}

static ssize_t ovl_splice_read(struct file *in, loff_t *ppos,

			 struct pipe_inode_info *pipe, size_t len,

			 unsigned int flags)

{

	ssize_t ret;

	struct fd real;

	const struct cred *old_cred;

	ret = ovl_real_fdget(in, &real);

	if (ret)

		return ret;

	old_cred = ovl_override_creds(file_inode(in)->i_sb);

	ret = generic_file_splice_read(real.file, ppos, pipe, len, flags);

	revert_creds(old_cred);

	ovl_file_accessed(in);

	fdput(real);

	return ret;

}

static ssize_t

ovl_splice_write(struct pipe_inode_info *pipe, struct file *out,

			  loff_t *ppos, size_t len, unsigned int flags)

{

	struct fd real;

	const struct cred *old_cred;

	ssize_t ret;

	ret = ovl_real_fdget(out, &real);

	if (ret)

		return ret;

	old_cred = ovl_override_creds(file_inode(out)->i_sb);

	ret = iter_file_splice_write(pipe, real.file, ppos, len, flags);

	revert_creds(old_cred);

	ovl_file_accessed(out);

	fdput(real);

	return ret;

}

static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)

{

	struct fd real;

	.fadvise	= ovl_fadvise,

	.unlocked_ioctl	= ovl_ioctl,

	.compat_ioctl	= ovl_compat_ioctl,

	.splice_read    = ovl_splice_read,

	.splice_write   = ovl_splice_write,

	.copy_file_range	= ovl_copy_file_range,

	.remap_file_range	= ovl_remap_file_range,