LoongArch: BPF: Make arch_protect_bpf_trampoline() return 0

Occasionally there exist "text_copy_cb: operation failed" when executing
the bpf selftests, the reason is copy_to_kernel_nofault() failed and the
ecode of ESTAT register is 0x4 (PME: Page Modification Exception) due to
the pte is not writeable. The root cause is that there is another place
to set the pte entry as readonly which is in the generic weak version of
arch_protect_bpf_trampoline().

There are two ways to fix this race condition issue: the direct way is
to modify the generic weak arch_protect_bpf_trampoline() to add a mutex
lock for set_memory_rox(), but the other simple and proper way is to
just make arch_protect_bpf_trampoline() return 0 in the arch-specific
code because LoongArch has already use the BPF prog pack allocator for
trampoline.

Here are the trimmed kernel log messages:

  copy_to_kernel_nofault: memory access failed, ecode 0x4
  copy_to_kernel_nofault: the caller is text_copy_cb+0x50/0xa0
  text_copy_cb: operation failed
  ------------[ cut here ]------------
  bpf_prog_pack bug: missing bpf_arch_text_invalidate?
  WARNING: kernel/bpf/core.c:1008 at bpf_prog_pack_free+0x200/0x228
  ...
  Call Trace:
  [<9000000000248914>] show_stack+0x64/0x188
  [<9000000000241308>] dump_stack_lvl+0x6c/0x9c
  [<90000000002705bc>] __warn+0x9c/0x200
  [<9000000001c428c0>] __report_bug+0xa8/0x1c0
  [<9000000001c42b5c>] report_bug+0x64/0x120
  [<9000000001c7dcd0>] do_bp+0x270/0x3c0
  [<9000000000246f40>] handle_bp+0x120/0x1c0
  [<900000000047b030>] bpf_prog_pack_free+0x200/0x228
  [<900000000047b2ec>] bpf_jit_binary_pack_free+0x24/0x60
  [<900000000026989c>] bpf_jit_free+0x54/0xb0
  [<900000000029e10c>] process_one_work+0x184/0x610
  [<900000000029ef8c>] worker_thread+0x24c/0x388
  [<90000000002a902c>] kthread+0x13c/0x170
  [<9000000001c7dfe8>] ret_from_kernel_thread+0x28/0x1c0
  [<9000000000246624>] ret_from_kernel_thread_asm+0xc/0x88

  ---[ end trace 0000000000000000 ]---

Here is a simple shell script to reproduce:

  #!/bin/bash

  for ((i=1; i<=1000; i++))
  do
    echo "Under testing $i ..."
    dmesg -c > /dev/null
    ./test_progs -t fentry_attach_stress > /dev/null
    dmesg -t | grep "text_copy_cb: operation failed"
    if [ $? -eq 0 ]; then
      break
    fi
  done

Cc: stable@vger.kernel.org
Fixes: 4ab17e76 ("LoongArch: BPF: Use BPF prog pack allocator")
Acked-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>