exfat: check return value of sb_min_blocksize in exfat_read_boot_sector

exfat: check return value of sb_min_blocksize in exfat_read_boot_sector

sb_min_blocksize() may return 0. Check its return value to avoid
accessing the filesystem super block when sb->s_blocksize is 0.

Cc: stable@vger.kernel.org # v6.15
Fixes: 719c1e1829166d ("exfat: add super block operations")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Yongpeng Yang <yangyongpeng@xiaomi.com>
Link: https://patch.msgid.link/20251104125009.2111925-3-yangyongpeng.storage@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>