exfat: fix divide-by-zero in exfat_allocate_bitmap

In the Linux kernel, the following vulnerability has been resolved: exfat: fix divide-by-zero in exfat_allocate_bitmap The variable max_ra_count can be 0 in exfat_allocate_bitmap(), which causes a divide-by-zero error in the subsequent modulo operation (i % max_ra_count), leading to a system crash. When max_ra_count is 0, it means that readahead is not used. This patch load the bitmap without readahead.