btrfs: print available space for a block group when dumping a space info

When dumping a space info, we iterate over all its block groups and then
print their size and the amounts of bytes used, reserved, pinned, etc.
When debugging -ENOSPC problems it's also useful to know how much space
is available (free), so calculate that and print it as well.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/space-info.c

@@ -523,13 +523,18 @@ void btrfs_dump_space_info(struct btrfs_fs_info *fs_info,
 	down_read(&info->groups_sem);
 again:
 	list_for_each_entry(cache, &info->block_groups[index], list) {
+		u64 avail;
+
 		spin_lock(&cache->lock);
+		avail = cache->length - cache->used - cache->pinned -
+			cache->reserved - cache->delalloc_bytes -
+			cache->bytes_super - cache->zone_unusable;
 		btrfs_info(fs_info,
-"block group %llu has %llu bytes, %llu used %llu pinned %llu reserved %llu delalloc %llu super %llu zone_unusable %s",
+"block group %llu has %llu bytes, %llu used %llu pinned %llu reserved %llu delalloc %llu super %llu zone_unusable (%llu bytes available) %s",
 			   cache->start, cache->length, cache->used, cache->pinned,
 			   cache->reserved, cache->delalloc_bytes,
 			   cache->bytes_super, cache->zone_unusable,
-			   cache->ro ? "[readonly]" : "");
+			   avail, cache->ro ? "[readonly]" : "");
 		spin_unlock(&cache->lock);
 		btrfs_dump_free_space(cache, bytes);
 	}