btrfs: add space reservation tracepoint for reserved bytes

I noticed when folding the trace_btrfs_space_reservation() tracepoint
into the btrfs_space_info_update_* helpers that we didn't emit a
tracepoint when doing btrfs_add_reserved_bytes().  I know this is
because we were swapping bytes_may_use for bytes_reserved, so in my mind
there was no reason to have the tracepoint there.  But now there is
because we always emit the unreserve for the bytes_may_use side, and
this would have broken if compression was on anyway.  Add a tracepoint
to cover the bytes_reserved counter so the math still comes out right.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/block-group.c

@@ -2758,6 +2758,8 @@ int btrfs_add_reserved_bytes(struct btrfs_block_group_cache *cache,
 	} else {
 		cache->reserved += num_bytes;
 		space_info->bytes_reserved += num_bytes;
+		trace_btrfs_space_reservation(cache->fs_info, "space_info",
+					      space_info->flags, num_bytes, 1);
 		btrfs_space_info_update_bytes_may_use(cache->fs_info,
 						      space_info, -ram_bytes);
 		if (delalloc)