mm: page_frag: fix a compile error when kernel is not compiled

page_frag test module is an out of tree module, but built
using KDIR as the main kernel tree, the mm test suite is
just getting skipped if newly added page_frag test module
fails to compile due to kernel not yet compiled.

Fix the above problem by ensuring both kernel is built first
and a newer kernel which has page_frag_cache.h is used.

CC: Andrew Morton <akpm@linux-foundation.org>
CC: Alexander Duyck <alexanderduyck@fb.com>
CC: Linux-MM <linux-mm@kvack.org>
Fixes: 7fef0dec ("mm: page_frag: add a test module for page_frag")
Fixes: 65941f10 ("mm: move the page fragment allocator from page_alloc into its own file")
Reported-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Tested-by: Mark Brown <broonie@kernel.org>
Link: https://patch.msgid.link/20241119033012.257525-1-linyunsheng@huawei.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>