bcachefs: Fix bch2_alloc_sectors_start_trans() error handling

When we fail to allocate because of insufficient open buckets, we don't
want to retry from the full set of devices - we just want to retry in
blocking mode.

But if the retry in blocking mode fails with a different error code, we
end up squashing the -BCH_ERR_open_buckets_empty error with an error
that makes us thing we won't be able to allocate (insufficient_devices)
- which is incorrect when we didn't try to allocate from the full set of
devices, and causes the write to fail.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>