mm/page_alloc: fix __alloc_size attribute for alloc_pages_exact_nid

The second parameter of alloc_pages_exact_nid is the one indicating the
size of memory pointed by the returned pointer.

Link: https://lkml.kernel.org/r/YbjEgwhn4bGblp//@coeus


Fixes: abd58f38 ("mm/page_alloc: add __alloc_size attributes for better bounds checking")
Signed-off-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Daniel Micay <danielmicay@gmail.com>
Cc: Levente Polyak <levente@leventepolyak.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>