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

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 <[email protected]>
CC: Alexander Duyck <[email protected]>
CC: Linux-MM <[email protected]>
Fixes: 7fef0dec415c ("mm: page_frag: add a test module for page_frag")
Fixes: 65941f10caf2 ("mm: move the page fragment allocator from page_alloc into its own file")
Reported-by: Mark Brown <[email protected]>
Signed-off-by: Yunsheng Lin <[email protected]>
Tested-by: Mark Brown <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>