samples: bpf: fix syscall_tp due to unused syscall

Currently, open() is called from the user program and it calls the syscall
'sys_openat', not the 'sys_open'. This leads to an error of the program
of user side, due to the fact that the counter maps are zero since no
function such 'sys_open' is called.

This commit adds the kernel bpf program which are attached to the
tracepoint 'sys_enter_openat' and 'sys_enter_openat'.

Fixes: 1da236b6

 ("bpf: add a test case for syscalls/sys_{enter|exit}_* tracepoints")
Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>