Btrfs: avoid races between super writeout and device list updates

On multi-device filesystems, btrfs writes supers to all of the devices
before considering a sync complete.  There wasn't any additional
locking between super writeout and the device list management code
because device management was done inside a transaction and
super writeout only happened  with no transation writers running.

With the btrfs fsync log and other async transaction updates, this
has been racey for some time.  This adds a mutex to protect
the device list.  The existing volume mutex could not be reused due to
transaction lock ordering requirements.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/volumes.h

@@ -96,7 +96,12 @@ struct btrfs_fs_devices {
 	u64 rw_devices;
 	u64 total_rw_bytes;
 	struct block_device *latest_bdev;
-	/* all of the devices in the FS */
+
+	/* all of the devices in the FS, protected by a mutex
+	 * so we can safely walk it to write out the supers without
+	 * worrying about add/remove by the multi-device code
+	 */
+	struct mutex device_list_mutex;
 	struct list_head devices;
 
 	/* devices not currently being allocated */