Make ftrace_regs a common trace interface for function entry/exit tracing
summary
Proposal replaces pt_regs with ftrace_regs for function entry/exit tracing, claiming a smaller, parameter-focused register set that avoids saving control registers and huge stacks on RISC (mips64 cited) and removes the need to copy into pt_regs for eBPF. Implementation converts fprobe/rethook to use the function-graph trampoline and per-task shadow stack, removes the kprobe max_active limit, and reports working integration while enabling eBPF programs to read registers directly from ftrace_regs.
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