vfs: verify source area in vfs_dedupe_file_range_one()

Call remap_verify_area() on the source file as well as the destination.

When called from vfs_dedupe_file_range() the check as already been
performed, but not so if called from layered fs (overlayfs, etc...)

Could ommit the redundant check in vfs_dedupe_file_range(), but leave for
now to get error early (for fear of breaking backward compatibility).

This call shouldn't be performance sensitive.
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/remap_range.c

@@ -456,8 +456,16 @@ loff_t vfs_dedupe_file_range_one(struct file *src_file, loff_t src_pos,
 	if (ret)
 		return ret;
 
+	/*
+	 * This is redundant if called from vfs_dedupe_file_range(), but other
+	 * callers need it and it's not performance sesitive...
+	 */
+	ret = remap_verify_area(src_file, src_pos, len, false);
+	if (ret)
+		goto out_drop_write;
+
 	ret = remap_verify_area(dst_file, dst_pos, len, true);
-	if (ret < 0)
+	if (ret)
 		goto out_drop_write;
 
 	ret = -EPERM;