erofs: fix error return code in erofs_read_superblock()

'ret' will be overwritten to 0 if erofs_sb_has_sb_chksum() return true,
thus 0 will return in some error handling cases. Fix to return negative
error code -EINVAL instead of 0.

Link: https://lore.kernel.org/r/20210519141657.3062715-1-weiyongjun1@huawei.com
Fixes: b858a484

 ("erofs: support superblock checksum")
Cc: stable <stable@vger.kernel.org> # 5.5+
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Gao Xiang <xiang@kernel.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <xiang@kernel.org>