selftests/bpf: Return true/false (not 1/0) from bool functions

Return boolean values ("true" or "false") instead of 1 or 0 from bool
functions.  This fixes the following warnings from coccicheck:

./tools/testing/selftests/bpf/progs/test_xdp_noinline.c:567:9-10: WARNING:
return of 0/1 in function 'get_packet_dst' with return type bool
./tools/testing/selftests/bpf/progs/test_l4lb_noinline.c:221:9-10: WARNING:
return of 0/1 in function 'get_packet_dst' with return type bool

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/1648779354-14700-1-git-send-email-baihaowen@meizu.com