Btrfs: rename root_times_lock to root_item_lock

Originally root_times_lock was introduced as part of send/receive
code however newly developed patch to label the subvol reused
the same lock, so renaming it for a meaningful name.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 6ba56ae..313a6ad 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1645,7 +1645,7 @@
 
 	int force_cow;
 
-	spinlock_t root_times_lock;
+	spinlock_t root_item_lock;
 };
 
 struct btrfs_ioctl_defrag_range_args {