kernel: 6.6: prevent garbage-cleanup of BPF kfuncs

kernel: 6.6: prevent garbage-cleanup of BPF kfuncs

Backport an upstream Linux fix to prevent BPF kfunc functions from being
removed during linker optimization when building BTF-enabled vmlinux.

The telltale sign of this occurring is build log warnings (e.g. mips64el):

      BTFIDS  vmlinux
    WARN: resolve_btfids: unresolved symbol bpf_verify_pkcs7_signature
    WARN: resolve_btfids: unresolved symbol bpf_lookup_user_key
    WARN: resolve_btfids: unresolved symbol bpf_lookup_system_key
    WARN: resolve_btfids: unresolved symbol bpf_key_put
    WARN: resolve_btfids: unresolved symbol bpf_iter_task_next
    WARN: resolve_btfids: unresolved symbol bpf_iter_css_task_new
    WARN: resolve_btfids: unresolved symbol bpf_get_file_xattr
    WARN: resolve_btfids: unresolved symbol bpf_ct_insert_entry
      NM      System.map
      SORTTAB vmlinux

Link: https://lore.kernel.org/bpf/ZlkoM6%2FPSxVcGM6X@kodidev-ubuntu/
Signed-off-by: Tony Ambardar <itugrok@yahoo.com>
Link: https://github.com/openwrt/openwrt/pull/15697
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>