bpf: support fsession for bpf_session_is_return

bpf: support fsession for bpf_session_is_return

If fsession exists, we will use the bit (1 << BPF_TRAMP_IS_RETURN_SHIFT)
in ((u64 *)ctx)[-1] to store the "is_return" flag.

The logic of bpf_session_is_return() for fsession is implemented in the
verifier by inline following code:

  bool bpf_session_is_return(void *ctx)
  {
      return (((u64 *)ctx)[-1] >> BPF_TRAMP_IS_RETURN_SHIFT) & 1;
  }

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Co-developed-by: Leon Hwang <leon.hwang@linux.dev>
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Link: https://lore.kernel.org/r/20260124062008.8657-5-dongml2@chinatelecom.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>