btrfs: qgroup: use qgroup_iterator in qgroup_convert_meta()

btrfs: qgroup: use qgroup_iterator in qgroup_convert_meta()

[ Upstream commit 0913445082496c2b29668ee26521401b273838b8 ]

With the new qgroup_iterator_add() and qgroup_iterator_clean(), we can
get rid of the ulist and its GFP_ATOMIC memory allocation.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Stable-dep-of: b321a52cce06 ("btrfs: free qgroup pertrans reserve on transaction abort")
Signed-off-by: Sasha Levin <sashal@kernel.org>