bcachefs: Fix early startup error path

bcachefs: Fix early startup error path

Don't set JOURNAL_running until we're also calling
journal_space_available() for the first time.

If JOURNAL_running is set, shutdown will write an empty journal entry -
but this will hit an assert in journal_entry_open() if we've never
called journal_space_available().

Reported-by: syzbot+53bb24d476ef8368a7f0@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>