xfs: fix error bailout in xrep_abt_build_new_trees

Dan Carpenter reports:

"Commit 4bdfd7d1 ("xfs: repair free space btrees") from Dec 15,
2023 (linux-next), leads to the following Smatch static checker
warning:

        fs/xfs/scrub/alloc_repair.c:781 xrep_abt_build_new_trees()
        warn: missing unwind goto?"

That's a bug, so let's fix it.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Fixes: 4bdfd7d1 ("xfs: repair free space btrees")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>