tracing: Fix the bug where bpf_get_stackid returns -EFAULT on the ARM64

When using bpf_program__attach_kprobe_multi_opts on ARM64 to hook a BPF program
that contains the bpf_get_stackid function, the BPF program fails
to obtain the stack trace and returns -EFAULT.

This is because ftrace_partial_regs omits the configuration of the pstate register,
leaving pstate at the default value of 0. When get_perf_callchain executes,
it uses user_mode(regs) to determine whether it is in kernel mode.
This leads to a misjudgment that the code is in user mode,
so perf_callchain_kernel is not executed and the function returns directly.
As a result, trace->nr becomes 0, and finally -EFAULT is returned.

Therefore, the assignment of the pstate register is added here.

Fixes: b9b55c8912ce ("tracing: Add ftrace_partial_regs() for converting ftrace_regs to pt_regs")
Closes: https://lore.kernel.org/bpf/20250919071902.554223-1-yangfeng59949@163.com/
Signed-off-by: Feng Yang <yangfeng@kylinos.cn>
Tested-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>

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