powerpc/spufs: fix out-of-bounds access in spufs_mem_mmap_access()

In the Linux kernel, the following vulnerability has been resolved: powerpc/spufs: fix out-of-bounds access in spufs_mem_mmap_access() spufs_mem_mmap_access() computes the local store offset as address - vma->vm_start, but bounds-checks it against vma->vm_end instead of the local store size. On 64-bit, offset is always well below vma->vm_end, so the clamp never fires and len stays unbounded against the LS_SIZE buffer returned by ctx->ops->get_ls(). Reject offsets at or beyond LS_SIZE and clamp len to the remaining space, mirroring the guard already used by spufs_mem_mmap_fault() and spufs_ps_fault().