selftests/bpf: Set the default value of consumer_cnt as 0

selftests/bpf: Set the default value of consumer_cnt as 0

Considering that only bench_ringbufs.c supports consumer, just set the
default value of consumer_cnt as 0. After that, update the validity
check of consumer_cnt, remove unused consumer_thread code snippets and
set consumer_cnt as 1 in run_bench_ringbufs.sh accordingly.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20230613080921.1623219-5-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>