mm/vmalloc: allocate small pages for area->pages

mm/vmalloc: allocate small pages for area->pages

The area->pages stores the struct pages allocated for vmalloc mappings.
The allocated memory can be hugepage if arch has HAVE_ARCH_HUGE_VMALLOC
set, while area->pages itself does not have to be hugepage backed.

Suppose that we want to vmalloc 1026M of memory, then area->pages is
2052K in size, which is large than PMD_SIZE when the pagesize is 4K.
Currently, 4096K will be allocated for area->pages, wherein 2044K is
wasted.

This introduces __vmalloc_node_no_huge, and makes area->pages backed by
small pages, because I think to allocate hugepage for area->pages is
unnecessary and vulnerable to abuse.

Link: https://lkml.kernel.org/r/dd04f516643fde4206c1fe93818526a768125c75.1638870169.git.xuyu@linux.alibaba.com
Signed-off-by: Xu Yu <xuyu@linux.alibaba.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Uladzislau Rezki <uladzislau.rezki@sony.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>