Btrfs: Fix compile warning around num_online_cpus() in a min statement

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

fs/btrfs/disk-io.c

@@ -1528,7 +1528,8 @@ struct btrfs_root *open_ctree(struct super_block *sb,
 	fs_info->btree_inode->i_ino = 1;
 	fs_info->btree_inode->i_nlink = 1;
 
-	fs_info->thread_pool_size = min(num_online_cpus() + 2, 8);
+	fs_info->thread_pool_size = min_t(unsigned long,
+					  num_online_cpus() + 2, 8);
 
 	INIT_LIST_HEAD(&fs_info->ordered_extents);
 	spin_lock_init(&fs_info->ordered_extent_lock);