bpf: Speed up htab lookups for u32/u64 keys

Four- and eight-byte keys are common enough to warrant specialized htab
lookups. Use jhash_1word() and jhash_2words() for these key sizes instead
of the generic jhash2() path.

For JITed BPF_MAP_TYPE_HASH lookups, htab_map_gen_lookup() also knows the
map's fixed key size. Select u32- and u64-specific lookup entry points so
the compiler can specialize both hashing and key comparison. Other key
sizes continue to use the generic lookup entry point.

This preserves the jhash2() result and lookup semantics while avoiding
generic length handling.

Measure this with six runs of:

  ./bench -w3 -d10 -a bpf-hashmap-lookup \
      --key_size KEY_SIZE --max_entries MAX_ENTRIES \
      --nr_entries NR_ENTRIES --nr_loops NR_LOOPS \
      --map_flags 0x40

inside a vng guest with two vCPUs and 4 GiB of RAM. Run the baseline
first and the optimized kernel second. Use these workloads:

             max_entries   nr_entries    nr_loops
  small              512          256   8,388,608
  medium          10,000        5,000   8,000,000
  large          100,000       50,000   8,000,000

Mean throughput in million lookups per second is:

  Baseline:
                         key size (bytes)
                      1        4        8       10
    small         89.45    91.21   103.40    91.74
    medium       125.97    80.25    91.94    78.52
    large        140.67    53.56    58.68    47.31

  Optimized:
                         key size (bytes)
                      1        4        8       10
    small         86.95   197.77   168.65    90.88
    medium       124.97   147.48   137.76    77.51
    large        131.06    78.51    76.04    47.25

  Change:
                         key size (bytes)
                      1        4        8       10
    small         -2.79% +116.83%  +63.11%   -0.94%
    medium        -0.79%  +83.78%  +49.83%   -1.28%
    large         -6.84%  +46.58%  +29.58%   -0.12%

The u32 specialization improves lookup throughput by 47-117%, and the
u64 specialization improves it by 30-63%. Ten-byte controls remain
within a 1.3% slowdown. One-byte controls range from -6.8% to -0.8%.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Anton Protopopov <a.s.protopopov@gmail.com>
Link: https://patch.msgid.link/20260922-hashtab_fast_hashfn-v3-1-b7b0e9bc33ac@meta.com

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