bcachefs: fast exit when darray_make_room failed

In downgrade_table_extra, the return value is needed. When it
return failed, we should exit immediately.

Fixes: 7773df19 ("bcachefs: metadata version bucket_stripe_sectors")
Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>