powernv/memtrace: use __GFP_ZERO with alloc_contig_pages()

alloc_contig_pages()->alloc_contig_range() now supports __GFP_ZERO, so
let's use that instead to resolve our TODO.

Link: https://lkml.kernel.org/r/20241203094732.200195-7-david@redhat.com


Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Naveen N Rao <naveen@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>