bpf: Use VM_MAP instead of VM_ALLOC for ringbuf

In the Linux kernel, the following vulnerability has been resolved:

bpf: Use VM_MAP instead of VM_ALLOC for ringbuf

After commit 2fd3fb0be1d1 ("kasan, vmalloc: unpoison VM_ALLOC pages

after mapping"), non-VM_ALLOC mappings will be marked as accessible

in __get_vm_area_node() when KASAN is enabled. But now the flag for

ringbuf area is VM_ALLOC, so KASAN will complain out-of-bound access

after vmap() returns. Because the ringbuf area is created by mapping

allocated pages, so use VM_MAP instead.

After the change, info in /proc/vmallocinfo also changes from

[start]-[end] 24576 ringbuf_map_alloc+0x171/0x290 vmalloc user

to

[start]-[end] 24576 ringbuf_map_alloc+0x171/0x290 vmap user