ovl: fix oopses in ovl_fill_super() failure paths

ovl_free_fs() dereferences ofs->workbasedir and ofs->upper_mnt in cases when
those might not have been initialized yet.

Fix the initialization order for these fields.

Reported-by:  <syzbot+c75f181dc8429d2eb887@syzkaller.appspotmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Cc:  <stable@vger.kernel.org> # v4.15
Fixes: 95e6d417 ("ovl: grab reference to workbasedir early")
Fixes: a9075cdb ("ovl: factor out ovl_free_fs() helper")