vfs: skip zero-length dedupe requests

Don't bother calling the filesystem for a zero-length dedupe request;
we can return zero and exit.
Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NAmir Goldstein <amir73il@gmail.com>
Signed-off-by: NDave Chinner <david@fromorbit.com>

fs/read_write.c

@@ -2009,6 +2009,11 @@ int vfs_dedupe_file_range_one(struct file *src_file, loff_t src_pos,
 	if (!dst_file->f_op->dedupe_file_range)
 		goto out_drop_write;
 
+	if (len == 0) {
+		ret = 0;
+		goto out_drop_write;
+	}
+
 	ret = dst_file->f_op->dedupe_file_range(src_file, src_pos,
 						dst_file, dst_pos, len);
 out_drop_write: