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 <[e-mail]>
CC: Alexander Duyck <[e-mail]>
CC: Linux-MM <[e-mail]>
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 <[e-mail]>
Signed-off-by: Yunsheng Lin <[e-mail]>
Tested-by: Mark Brown <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

tools/testing/selftests/mm/Makefile

···
 CFLAGS = -Wall -I $(top_srcdir) $(EXTRA_CFLAGS) $(KHDR_INCLUDES) $(TOOLS_INCLUDES)
 LDLIBS = -lrt -lpthread -lm
 
+KDIR ?= /lib/modules/$(shell uname -r)/build
+ifneq (,$(wildcard $(KDIR)/Module.symvers))
+ifneq (,$(wildcard $(KDIR)/include/linux/page_frag_cache.h))
 TEST_GEN_MODS_DIR := page_frag
+else
+PAGE_FRAG_WARNING = "missing page_frag_cache.h, please use a newer kernel"
+endif
+else
+PAGE_FRAG_WARNING = "missing Module.symvers, please have the kernel built first"
+endif
 
 TEST_GEN_FILES = cow
 TEST_GEN_FILES += compaction_test
···
 warn_missing_liburing:
 	@echo ; \
 	echo "Warning: missing liburing support. Some tests will be skipped." ; \
+	echo
+endif
+
+ifneq ($(PAGE_FRAG_WARNING),)
+all: warn_missing_page_frag
+
+warn_missing_page_frag:
+	@echo ; \
+	echo "Warning: $(PAGE_FRAG_WARNING). page_frag test will be skipped." ; \
 	echo
 endif

tools/testing/selftests/mm/page_frag/Makefile

···
 PAGE_FRAG_TEST_DIR := $(realpath $(dir $(abspath $(lastword $(MAKEFILE_LIST)))))
-KDIR ?= $(abspath $(PAGE_FRAG_TEST_DIR)/../../../../..)
+KDIR ?= /lib/modules/$(shell uname -r)/build
 
 ifeq ($(V),1)
 Q =