bcachefs: __journal_entry_close() never fails

bcachefs: __journal_entry_close() never fails

Previous patch just moved responsibility for incrementing the journal
sequence number and initializing the new journal entry from
__journal_entry_close() to journal_entry_open(); this patch makes the
analagous change for journal reservation state, incrementing the index
into array of journal_bufs at open time.

This means that __journal_entry_close() never fails to close an open
journal entry, which is important for the next patch that will change
our emergency shutdown behaviour.

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