bcachefs: bch2_journal_halt() now takes journal lock

bcachefs: bch2_journal_halt() now takes journal lock

This change is prep work for moving some work from
__journal_entry_close() to journal_entry_open(): without this change,
journal_entry_open() doesn't know if it's going to be able to open a new
journal entry until the cmpxchg loop, meaning it can't create the new
journal pin entry and update other global state because those have to be
done prior to the cmpxchg opening the new journal entry.

Fortunately, we don't call bch2_journal_halt() from interrupt context.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>