btrfs: always clear PERTRANS metadata during commit

[ Upstream commit 6e68de0bb0ed59e0554a0c15ede7308c47351e2d ]

It is possible to clear a root's IN_TRANS tag from the radix tree, but
not clear its PERTRANS, if there is some error in between. Eliminate
that possibility by moving the free up to where we clear the tag.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>