erofs: make erofs_{err,info}() support NULL sb parameter

Make erofs_err() and erofs_info() support NULL sb parameter for more
general usage.

Suggested-by: Gao Xiang <xiang@kernel.org>
Signed-off-by: Chunhai Guo <guochunhai@vivo.com>
Link: https://lore.kernel.org/r/20240103123202.3054718-1-guochunhai@vivo.com
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>