bpf: Minor improvements for bpf_cmp.

bpf: Minor improvements for bpf_cmp.

Few minor improvements for bpf_cmp() macro:
. reduce number of args in __bpf_cmp()
. rename NOFLIP to UNLIKELY
. add a comment about 64-bit truncation in "i" constraint
. use "ri" constraint for sizeof(rhs) <= 4
. improve error message for bpf_cmp_likely()

Before:
progs/iters_task_vma.c:31:7: error: variable 'ret' is uninitialized when used here [-Werror,-Wuninitialized]
   31 |                 if (bpf_cmp_likely(seen, <==, 1000))
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../bpf/bpf_experimental.h:325:3: note: expanded from macro 'bpf_cmp_likely'
  325 |                 ret;
      |                 ^~~
progs/iters_task_vma.c:31:7: note: variable 'ret' is declared here
../bpf/bpf_experimental.h:310:3: note: expanded from macro 'bpf_cmp_likely'
  310 |                 bool ret;
      |                 ^

After:
progs/iters_task_vma.c:31:7: error: invalid operand for instruction
   31 |                 if (bpf_cmp_likely(seen, <==, 1000))
      |                     ^
../bpf/bpf_experimental.h:324:17: note: expanded from macro 'bpf_cmp_likely'
  324 |                         asm volatile("r0 " #OP " invalid compare");
      |                                      ^
<inline asm>:1:5: note: instantiated into assembly here
    1 |         r0 <== invalid compare
      |            ^

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20240112220134.71209-1-alexei.starovoitov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>