Btrfs: copy string correctly in INO_LOOKUP ioctl

Memory areas [ptr, ptr+total_len] and [name, name+total_len]
may overlap, so it's wrong to use memcpy().

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index fd252ff..2bb0886 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -1757,11 +1757,10 @@
 		key.objectid = key.offset;
 		key.offset = (u64)-1;
 		dirid = key.objectid;
-
 	}
 	if (ptr < name)
 		goto out;
-	memcpy(name, ptr, total_len);
+	memmove(name, ptr, total_len);
 	name[total_len]='\0';
 	ret = 0;
 out: