Btrfs: Fix typo in fs/btrfs

Correct spelling typo in btrfs.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index cad1656..2d41cb2 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -413,7 +413,7 @@
 	__le64 bytes_used;
 	__le64 num_devices;
 	/* future */
-	__le64 unsed_64[4];
+	__le64 unused_64[4];
 
 	u8 tree_root_level;
 	u8 chunk_root_level;