btrfs: Remove unused readahead spinlock

The reada_lock in struct btrfs_device was only initialised, and not
actually used.  That's good because there's another lock also called
reada_lock in the btrfs_fs_info that was quite heavily used.  Remove
this one.
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -319,7 +319,6 @@ static struct btrfs_device *__alloc_device(void)
 
 	spin_lock_init(&dev->io_lock);
 
-	spin_lock_init(&dev->reada_lock);
 	atomic_set(&dev->reada_in_flight, 0);
 	atomic_set(&dev->dev_stats_ccnt, 0);
 	btrfs_device_data_ordered_init(dev);

fs/btrfs/volumes.h

@@ -137,7 +137,6 @@ struct btrfs_device {
 	struct rcu_head rcu;
 
 	/* readahead state */
-	spinlock_t reada_lock;
 	atomic_t reada_in_flight;
 	u64 reada_next;
 	struct reada_zone *reada_curr_zone;