selftests/bpf: Fix a tautological-constant-out-of-range-compare compiler warning

When using clang to build selftests with LLVM=1 in make commandline,
I hit the following compiler warning:

  benchs/bench_bloom_filter_map.c:84:46: warning: result of comparison of constant 256
    with expression of type '__u8' (aka 'unsigned char') is always false
    [-Wtautological-constant-out-of-range-compare]
                if (args.value_size < 2 || args.value_size > 256) {
                                           ~~~~~~~~~~~~~~~ ^ ~~~

The reason is arg.vaue_size has type __u8, so comparison "args.value_size > 256"
is always false.

This patch fixed the issue by doing proper comparison before assigning the
value to args.value_size. The patch also fixed the same issue in two
other places.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211112204838.3579953-1-yhs@fb.com