CVE-2025-39793 io_uring/memmap: cast nr_pages to size_t before shifting

In the Linux kernel, the following vulnerability has been resolved: iouring/memmap: cast nrpages to sizet before shifting If the allocated size exceeds UINTMAX, then it's necessary to cast the mr-nrpages value to sizet to prevent it from overflowing. In practice this isn't much of a concern as th...