Btrfs: make BUG_ON() in add_pinned_bytes() an ASSERT()

The value of flags is one of DATA/METADATA/SYSTEM, they must exist at
when add_pinned_bytes is called.
Signed-off-by: Omar Sandoval <osandov@fb.com>
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ added changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -783,7 +783,7 @@ static void add_pinned_bytes(struct btrfs_fs_info *fs_info, s64 num_bytes,
 	}
 
 	space_info = __find_space_info(fs_info, flags);
-	BUG_ON(!space_info); /* Logic bug */
+	ASSERT(space_info);
 	percpu_counter_add(&space_info->total_bytes_pinned, num_bytes);
 }