vfs: export vfs_dedupe_file_range_one() to modules

This is needed by the stacked dedupe implementation in overlayfs.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

include/linux/fs.h

@@ -1829,6 +1829,10 @@ extern int vfs_dedupe_file_range_compare(struct inode *src, loff_t srcoff,
 					 loff_t len, bool *is_same);
 extern int vfs_dedupe_file_range(struct file *file,
 				 struct file_dedupe_range *same);
+extern int vfs_dedupe_file_range_one(struct file *src_file, loff_t src_pos,
+				     struct file *dst_file, loff_t dst_pos,
+				     u64 len);
+
 
 struct super_operations {
    	struct inode *(*alloc_inode)(struct super_block *sb);