isofs: check the return value of sb_min_blocksize() in isofs_fill_super

isofs: check the return value of sb_min_blocksize() in isofs_fill_super

sb_min_blocksize() may return 0. Check its return value to avoid
opt->blocksize and sb->s_blocksize is 0.

Cc: stable@vger.kernel.org # v6.15
Fixes: 1b17a46c9243e9 ("isofs: convert isofs to use the new mount API")
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Yongpeng Yang <yangyongpeng@xiaomi.com>
Link: https://patch.msgid.link/20251104125009.2111925-4-yangyongpeng.storage@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>