powerpc64/bpf: Support tailcalls with subprogs

powerpc64/bpf: Support tailcalls with subprogs

Enable tailcalls support in subprogs by passing tail call count as
reference instead of value. The actual tailcall count is always
maintained in the tailcall field present in the frame of main
function (also called entry function). The tailcall field in the
stack frame of subprogs contains reference to the tailcall field
in the stack frame of main BPF program. Accordingly, rename
tail_call_cnt field in the stack layout to tail_call_info.

Signed-off-by: Abhishek Dubey <adubey@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260124075223.6033-3-adubey@linux.ibm.com