bpf: Exhaustive test coverage for signed division and modulo

bpf: Exhaustive test coverage for signed division and modulo

Extend lib/test_bpf.c to provide comprehensive test coverage for BPF
signed division (SDIV) and signed modulo (SMOD) instructions, both
32-bit and 64-bit variants with immediate operands.

Introduce F_ALU32 and F_SIGNED flags to replace the less readable
bool alu32 and s16 off parameters throughout the test helpers. The
BPF instruction 'off' field is derived from flags only at the point
of instruction encoding.

Changes:
- Add enum { F_ALU32 = 1, F_SIGNED = 2 } for readable test flags.
- __bpf_alu_result(): take u32 flags instead of separate signed/alu32
  parameters. Narrows operands internally for ALU32 (unsigned via u32
  cast, signed via s32 cast) before computing the reference result.
- __bpf_emit_alu64_imm(), __bpf_emit_alu32_imm(): pass flags through
  to __bpf_alu_result, derive 'off' for instruction encoding locally.
- __bpf_fill_alu_imm_regs(): take u32 flags, use F_ALU32/F_SIGNED for
  operand setup and single-line __bpf_alu_result() call.
- __bpf_fill_alu_shift(), __bpf_fill_alu_shift_same_reg(): convert
  bool alu32 parameter to u32 flags for consistency.
- New test fill functions: bpf_fill_alu{32,64}_{sdiv,smod}_imm() and
  bpf_fill_alu{32,64}_{sdiv,smod}_imm_regs(), each testing all
  immediate value magnitudes and all register pair combinations.
- All existing unsigned tests updated to use flags (0 or F_ALU32),
  preserving backward compatibility.

8 new test cases added:
  ALU64_SDIV_K, ALU64_SMOD_K (immediate magnitudes + register combos)
  ALU32_SDIV_K, ALU32_SMOD_K (immediate magnitudes + register combos)

Test results:
  test_bpf: Summary: 1061 PASSED, 0 FAILED, [1049/1049 JIT'ed]
  test_bpf: test_tail_calls: Summary: 10 PASSED, 0 FAILED, [10/10 JIT'ed]
  test_bpf: test_skb_segment: Summary: 2 PASSED, 0 FAILED

Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Jie Meng <jmeng@fb.com>
Link: https://lore.kernel.org/r/20260413172311.3918767-1-jmeng@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>