LoongArch: BPF: Inline bpf_get_smp_processor_id() helper

LoongArch: BPF: Inline bpf_get_smp_processor_id() helper

The pointer to thread_info is always available in the $tp register, so
the call to bpf_get_smp_processor_id() can be inlined into a single load
instruction.

(1) Here is the sample test.c:

  #include <linux/bpf.h>
  #include <bpf/bpf_helpers.h>

  SEC("raw_tp/sys_enter")
  int test_cpuid(void *ctx)
  {
	  return bpf_get_smp_processor_id();
  }
  char _license[] SEC("license") = "GPL";

(2) Here are the test steps:

  sudo yum install libbpf-devel kernel-devel bpftool
  clang -target bpf -O2 -c test.c -o test.o
  sudo sysctl -w net.core.bpf_jit_enable=1
  sudo bpftool prog show name test_cpuid
  sudo rm -f /sys/fs/bpf/test_cpuid
  sudo bpftool prog load test.o /sys/fs/bpf/test_cpuid
  ID=$(sudo bpftool prog show pinned /sys/fs/bpf/test_cpuid | grep -oE '^[0-9]+')
  sudo bpftool prog dump jited id $ID

(3) Here are the test results:

Before: 6 instructions

  ...
  64:	lu12i.w	$t1, 1213
  68:	ori	$t1, $t1, 1680
  6c:	lu32i.d	$t1, 0
  70:	lu52i.d	$t1, $t1, -1792
  74:	jirl	$ra, $t1, 0
  78:	move	$a5, $a0
  ...

After: 1 instruction

  ...
  64:	ld.wu	$a5, $tp, 16
  ...

This is similar with commit 2ddec2c80b44 ("riscv, bpf: inline
bpf_get_smp_processor_id()").

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>