Use $(KHDR_INCLUDES) as lookup path for kernel headers. This prevents
building against kernel headers from the build environment in scenarios
where kernel headers are installed into a specific output directory
(O=...).

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>
Cc: <stable@vger.kernel.org> # 5.18+
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

epoll build fails to find pthread lib. Fix Makefile to use LDLIBS
instead of LDFLAGS. LDLIBS is the right flag to use here with -l
option when invoking ld.

gcc -I../../../../../usr/include/  -lpthread  epoll_wakeup_test.c  -o .../tools/testing/selftests/filesystems/epoll/epoll_wakeup_test
/usr/bin/ld: /tmp/ccaZvJUl.o: in function `kill_timeout':
epoll_wakeup_test.c:(.text+0x4dd): undefined reference to `pthread_kill'
/usr/bin/ld: epoll_wakeup_test.c:(.text+0x4f2): undefined reference to `pthread_kill'
/usr/bin/ld: /tmp/ccaZvJUl.o: in function `epoll9':
epoll_wakeup_test.c:(.text+0x6382): undefined reference to `pthread_create'
/usr/bin/ld: epoll_wakeup_test.c:(.text+0x64d2): undefined reference to `pthread_create'
/usr/bin/ld: epoll_wakeup_test.c:(.text+0x6626): undefined reference to `pthread_join'
/usr/bin/ld: epoll_wakeup_test.c:(.text+0x684c): undefined reference to `pthread_tryjoin_np'
/usr/bin/ld: epoll_wakeup_test.c:(.text+0x6864): undefined reference to `pthread_kill'
/usr/bin/ld: epoll_wakeup_test.c:(.text+0x6878): undefined reference to `pthread_join'

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

This adds the promised selftest for epoll.  It will verify the wakeups
of epoll.  Including leaf and nested mode, epoll_wait() and poll() and
multi-threads.

Link: http://lkml.kernel.org/r/20191009121518.4027-1-r@hev.cc
Signed-off-by: hev <r@hev.cc>
Reviewed-by: Roman Penyaev <rpenyaev@suse.de>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>