fs/proc/task_mmu: prevent integer overflow in pagemap_scan_get_args()

In the Linux kernel, the following vulnerability has been resolved: fs/proc/task_mmu: prevent integer overflow in pagemap_scan_get_args() The "arg->vec_len" variable is a u64 that comes from the user at the start of the function. The "arg->vec_len * sizeof(struct page_region))" multiplication can lead to integer wrapping. Use size_mul() to avoid that. Also the size_add/mul() functions work on unsigned long so for 32bit systems we need to ensure that "arg->vec_len" fits in an unsigned long.