bcachefs: Fix bch2_dev_journal_alloc() spuriously failing

Previously, we fixed journal resize spuriousl failing with
-BCH_ERR_open_buckets_empty, but initial journal allocation was missed
because it didn't invoke the "block on allocator" loop at all.

Factor out the "loop on allocator" code to fix that.

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