selftests/bpf: Add test for large offset bpf-to-bpf call

selftests/bpf: Add test for large offset bpf-to-bpf call

Add a selftest to verify the verifier and JIT behavior when handling
bpf-to-bpf calls with relative jump offsets exceeding the s16 boundary.

The test utilizes an inline assembly block with ".rept 32765" to generate
a massive dummy subprogram. By placing this padding between the main
program and the target subprogram, it forces the verifier to process a
bpf-to-bpf call where the imm field exceeds the s16 range.

- When JIT is enabled, it asserts that the program is successfully loaded
  and executes correctly to return the expected value. Since the fix
  does not change the JIT behavior, the test passes whether the fix is
  applied or not.
- When JIT is disabled, it also asserts that the program is successfully
  loaded and executes correctly to return the expected value 3.
  - Before the fix, the verifier rewrites the call instruction with a
    truncated offset (here 32768 -> -32768) and lets it pass. When the
    program is executed, the call instruction will go to a wrong target
    (the landing pad) instead of the intended subprogram, then return -1
    and fail.
  - After the fix, the verifier correctly handles the large offset and
    allows it to pass. The program then executes correctly to return the
    expected value 3.

Co-developed-by: Tianci Cao <ziye@zju.edu.cn>
Signed-off-by: Tianci Cao <ziye@zju.edu.cn>
Co-developed-by: Shenghao Yuan <shenghaoyuan0928@163.com>
Signed-off-by: Shenghao Yuan <shenghaoyuan0928@163.com>
Signed-off-by: Yazhou Tang <tangyazhou518@outlook.com>
Acked-by: Xu Kuohai <xukuohai@huawei.com>
Link: https://lore.kernel.org/r/20260506094714.419842-4-tangyazhou@zju.edu.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>