selftests/bpf: Move generated test files to $(TEST_GEN_FILES)

mainline inclusion
from mainline-5.12-rc1
commit d6ac8cad
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5EUVD
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d6ac8cad50f0c0fed5cfeb61301bb19e8e88985c

-------------------------------------------------

During an out-of-tree build, attempting to install the $(TEST_FILES)
into the $(OUTPUT) directory fails, because the objects were already
generated into $(OUTPUT):

rsync: [sender] link_stat "tools/testing/selftests/bpf/test_lwt_ip_encap.o" failed: No such file or directory (2)
rsync: [sender] link_stat "tools/testing/selftests/bpf/test_tc_edt.o" failed: No such file or directory (2)

Use $(TEST_GEN_FILES) instead.
Signed-off-by: NJean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210113163319.1516382-4-jean-philippe@linaro.org
(cherry picked from commit d6ac8cad)
Signed-off-by: NWang Yufen <wangyufen@huawei.com>

Conflicts:
	tools/testing/selftests/bpf/Makefile
Signed-off-by: NWang Yufen <wangyufen@huawei.com>

tools/testing/selftests/bpf/Makefile

@@ -45,8 +45,7 @@ ifneq ($(BPF_GCC),)
 TEST_GEN_PROGS += test_progs-bpf_gcc
 endif
 
-TEST_GEN_FILES =
-TEST_FILES = test_lwt_ip_encap.o \
+TEST_GEN_FILES = test_lwt_ip_encap.o \
 	test_tc_edt.o
 
 # Order correspond to 'make run_tests' order