bpf: Fix test_obj_id.c for llvm 5.0

llvm 5.0 does not like the section name and the function name
to be the same:

clang -I. -I./include/uapi -I../../../include/uapi \
-I../../../../samples/bpf/ \
-Wno-compare-distinct-pointer-types \
-O2 -target bpf -c \
linux/tools/testing/selftests/bpf/test_obj_id.c -o \
linux/tools/testing/selftests/bpf/test_obj_id.o
fatal error: error in backend: 'test_prog_id' label emitted multiple times to
assembly file
clang-5.0: error: clang frontend command failed with exit code 70 (use -v to
see invocation)
clang version 5.0.0 (trunk 304326) (llvm/trunk 304329)

This patch makes changes to the section name and the function name.

Fixes: 95b9afd3987f ("bpf: Test for bpf ID")
Reported-by: Alexei Starovoitov <[e-mail]>
Reported-by: Yonghong Song <[e-mail]>
Signed-off-by: Martin KaFai Lau <[e-mail]>
Acked-by: Daniel Borkmann <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

tools/testing/selftests/bpf/test_obj_id.c

···
 	.max_entries = 1,
 };
 
-SEC("test_prog_id")
-int test_prog_id(struct __sk_buff *skb)
+SEC("test_obj_id_dummy")
+int test_obj_id(struct __sk_buff *skb)
 {
 	__u32 key = 0;
 	__u64 *value;