LoongArch: BPF: Support 64-bit pointers to kfuncs

Like commit 1cf3bfc60f ("bpf: Support 64-bit pointers to kfuncs")
for s390x, add support for 64-bit pointers to kfuncs for LoongArch.
Since the infrastructure is already implemented in BPF core, the only
thing need to be done is to override bpf_jit_supports_far_kfunc_call().

Before this change, several test_verifier tests failed:

  # ./test_verifier | grep # | grep FAIL
  #119/p calls: invalid kfunc call: ptr_to_mem to struct with non-scalar FAIL
  #120/p calls: invalid kfunc call: ptr_to_mem to struct with nesting depth > 4 FAIL
  #121/p calls: invalid kfunc call: ptr_to_mem to struct with FAM FAIL
  #122/p calls: invalid kfunc call: reg->type != PTR_TO_CTX FAIL
  #123/p calls: invalid kfunc call: void * not allowed in func proto without mem size arg FAIL
  #124/p calls: trigger reg2btf_ids[reg->type] for reg->type > __BPF_REG_TYPE_MAX FAIL
  #125/p calls: invalid kfunc call: reg->off must be zero when passed to release kfunc FAIL
  #126/p calls: invalid kfunc call: don't match first member type when passed to release kfunc FAIL
  #127/p calls: invalid kfunc call: PTR_TO_BTF_ID with negative offset FAIL
  #128/p calls: invalid kfunc call: PTR_TO_BTF_ID with variable offset FAIL
  #129/p calls: invalid kfunc call: referenced arg needs refcounted PTR_TO_BTF_ID FAIL
  #130/p calls: valid kfunc call: referenced arg needs refcounted PTR_TO_BTF_ID FAIL
  #486/p map_kptr: ref: reference state created and released on xchg FAIL

This is because the kfuncs in the loaded module are far away from
__bpf_call_base:

  ffff800002009440 t bpf_kfunc_call_test_fail1    [bpf_testmod]
  9000000002e128d8 T __bpf_call_base

The offset relative to __bpf_call_base does NOT fit in s32, which breaks
the assumption in BPF core. Enable bpf_jit_supports_far_kfunc_call() lifts
this limit.

Note that to reproduce the above result, tools/testing/selftests/bpf/config
should be applied, and run the test with JIT enabled, unpriv BPF enabled.

With this change, the test_verifier tests now all passed:

  # ./test_verifier
  ...
  Summary: 777 PASSED, 0 SKIPPED, 0 FAILED

Tested-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/net/bpf_jit.c

@@ -201,6 +201,11 @@ bool bpf_jit_supports_kfunc_call(void)
 	return true;
 }
 
+bool bpf_jit_supports_far_kfunc_call(void)
+{
+	return true;
+}
+
 /* initialized on the first pass of build_body() */
 static int out_offset = -1;
 static int emit_bpf_tail_call(struct jit_ctx *ctx)