tools/bpf: selftests: add map lookup to test_map_in_map bpf prog

The bpf_map_lookup_elem is added in the bpf program.
Without previous patch, the test change will trigger the
following error:
  $ ./test_maps
  ...
  ; value_p = bpf_map_lookup_elem(map, &key);
  20: (bf) r1 = r7
  21: (bf) r2 = r8
  22: (85) call bpf_map_lookup_elem#1
  ; if (!value_p || *value_p != 123)
  23: (15) if r0 == 0x0 goto pc+16
   R0=map_value(id=2,off=0,ks=4,vs=4,imm=0) R6=inv1 R7=map_ptr(id=0,off=0,ks=4,vs=4,imm=0)
   R8=fp-8,call_-1 R10=fp0,call_-1 fp-8=mmmmmmmm
  ; if (!value_p || *value_p != 123)
  24: (61) r1 = *(u32 *)(r0 +0)
   R0=map_value(id=2,off=0,ks=4,vs=4,imm=0) R6=inv1 R7=map_ptr(id=0,off=0,ks=4,vs=4,imm=0)
   R8=fp-8,call_-1 R10=fp0,call_-1 fp-8=mmmmmmmm
  bpf_spin_lock cannot be accessed directly by load/store

With the kernel fix in the previous commit, the error goes away.

Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>