selftests: memfd: Use installed kernel headers search path

Use $(KHDR_INCLUDES) as lookup path for installed kernel headers rather
than using kernel headers in include/uapi from the source kernel tree
kernel headers.

Remove bogus ../../../../include/ from the search path, because
kernel source headers are not needed by those user-space selftests, and
it causes issues because -I paths are searched before -isystem paths,
and conflicts for files appearing both in kernel sources and in uapi
headers with incompatible semantics (e.g. types.h).
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: linux-kselftest@vger.kernel.org
Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/memfd/Makefile

 # SPDX-License-Identifier: GPL-2.0
 CFLAGS += -D_FILE_OFFSET_BITS=64
-CFLAGS += -I../../../../include/uapi/
-CFLAGS += -I../../../../include/
-CFLAGS += -I../../../../usr/include/
+CFLAGS += $(KHDR_INCLUDES)
 
 TEST_GEN_PROGS := memfd_test
 TEST_PROGS := run_fuse_test.sh run_hugetlbfs_test.sh