Merge branch 'bpf-x86-inline-bpf_get_current_task-for-x86_64'

Menglong Dong says:

====================
bpf, x86: inline bpf_get_current_task() for x86_64

Inline bpf_get_current_task() and bpf_get_current_task_btf() for x86_64
to obtain better performance, and add the testcase for it.

Changes since v5:
* remove unnecessary 'ifdef' and __description in the selftests
* v5: https://lore.kernel.org/bpf/20260119070246.249499-1-dongml2@chinatelecom.cn/

Changes since v4:
* don't support the !CONFIG_SMP case
* v4: https://lore.kernel.org/bpf/20260112104529.224645-1-dongml2@chinatelecom.cn/

Changes since v3:
* handle the !CONFIG_SMP case
* ignore the !CONFIG_SMP case in the testcase, as we enable CONFIG_SMP
  for x86_64 in the selftests

Changes since v2:
* implement it in the verifier with BPF_MOV64_PERCPU_REG() instead of in
  x86_64 JIT (Alexei).

Changes since v1:
* add the testcase
* remove the usage of const_current_task
====================

Link: https://patch.msgid.link/20260120070555.233486-1-dongml2@chinatelecom.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>