Merge branch 'bpf-fix-builds-broken-by-fatal-resolve_btfids-warnings'

Sasha Levin says:

====================
bpf: Fix builds broken by fatal resolve_btfids warnings

Commit 201a454df948 ("resolve_btfids: Fix the --fatal_warnings exit
status") makes resolve_btfids warnings fail the build again with
CONFIG_WERROR=y. That exposed BTF ID references to functions some
configs don't build. KernelCI has been seeing i386
i386_defconfig+kselftest builds fail on linux-next since next-20261015:

  WARN: resolve_btfids: unresolved symbol bpf_arena_reserve_pages
  WARN: resolve_btfids: unresolved symbol bpf_arena_free_pages
  WARN: resolve_btfids: unresolved symbol bpf_arena_alloc_pages
  Failed to generate BTF for vmlinux
  Try to disable CONFIG_DEBUG_INFO_BTF

https://files.kernelci.org/kbuild-gcc-14-i386-kselftest-6ad2315ca3f195e1aa930c8a/build.log.gz

Patch 1 drops three special_kfunc_list entries that have no users,
bpf_arena_reserve_pages among them. Patch 2 makes the remaining arena
entries depend on the condition arena.o is built with. Patch 3 fixes the
same problem for bpf_lsm_socket_sendmsg in bpf_ksock.c, which breaks
x86_64 builds with CONFIG_BPF_LSM=y and CONFIG_SECURITY_NETWORK=n.

The entries left in special_kfunc_list resolve to the same IDs as
before, and the IDs that become BTF_ID_UNUSED were already zero.

Tested by building vmlinux with CONFIG_DEBUG_INFO_BTF=y and
CONFIG_WERROR=y for i386, x86_64, and x86_64 with CONFIG_BPF_LSM=y and
both CONFIG_SECURITY_NETWORK=n and =y, and comparing special_kfunc_list
and bpf_lsm_socket_sendmsg_id before and after the series.
====================

Link: https://patch.msgid.link/20261019211006.2574350-1-sashal@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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