os/FileJournal: return errors on make_writeable() if reopen fails

This is why #7738 is resulting in a crash instead of an error.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1487 from ceph/wip-7738

os/FileJournal: return errors on make_writeable() if reopen fails

Reviewed-by: Samuel Just <sam.just@inktank.com>