selftests/bpf: Isolate fmod_ret hooks by pid

modify_return's fmod_ret programs can override bpf_modify_return_test()'s
return value, which conflicts with get_func_ip_test when selftests run in
parallel.

Store current tgid in BSS and make modify_return hooks act only for that
tgid. For other tasks, fentry/fexit become no-ops and fmod_ret returns the
original ret.

Drop the serial-only restriction and remove the TODO comment.

Tested:
  sudo ./test_progs -t modify_return
  sudo ./test_progs -t get_func_ip_test
  sudo ./test_progs -j$(nproc) -t modify_return
  sudo ./test_progs -j$(nproc) -t get_func_ip_test

Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
Link: https://lore.kernel.org/r/20260313034540.255805-1-sun.jian.kdev@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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