bpf: add a test case to test single tp multiple bpf attachment

bpf: add a test case to test single tp multiple bpf attachment

The bpf sample program syscall_tp is modified to
show attachment of more than bpf programs
for a particular kernel tracepoint.

Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>