lib/test_meminit: allocate pages up to order MAX_ORDER

test_pages() tests the page allocator by calling alloc_pages() with
different orders up to order 10.

However, different architectures and platforms support different maximum
contiguous allocation sizes.  The default maximum allocation order
(MAX_ORDER) is 10, but architectures can use CONFIG_ARCH_FORCE_MAX_ORDER
to override this.  On platforms where this is less than 10, test_meminit()
will blow up with a WARN().  This is expected, so let's not do that.

Replace the hardcoded "10" with the MAX_ORDER macro so that we test
allocations up to the expected platform limit.

Link: https://lkml.kernel.org/r/20230714015238.47931-1-ajd@linux.ibm.com
Fixes: 5015a300

 ("lib: introduce test_meminit module")
Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Cc: Xiaoke Wang <xkernel.wang@foxmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>