btrfs: embed extent_changeset::range_changed to the structure

We can embed range_changed to the extent changeset to address following
problems:

- no need to allocate ulist dynamically, we also get rid of the GFP_NOFS
  for free
- fix lack of allocation failure checking in btrfs_qgroup_reserve_data

The stack consuption where extent_changeset is used slightly increases:

before: 16
after: 16 - 8 (for pointer) + 32 (sizeof ulist) = 40

Which is bearable.

Reviewed-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 9df6ed3..9140847 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -144,7 +144,7 @@
 	if (!set && (state->state & bits) == 0)
 		return;
 	changeset->bytes_changed += state->end - state->start + 1;
-	ret = ulist_add(changeset->range_changed, state->start, state->end,
+	ret = ulist_add(&changeset->range_changed, state->start, state->end,
 			GFP_ATOMIC);
 	/* ENOMEM */
 	BUG_ON(ret < 0);