kbuild: rpm-pkg: Preserve BTF sections in kernel modules during debuginfo stripping

After switching to the kernel's default package scripts for our local
kernel RPM builds, we noticed that module BTF entries were missing:

  $ ls /sys/kernel/btf/
  vmlinux    <<<< only vmlinux, no module BTF

Root cause: find-debuginfo.sh (from the debugedit package) prefers
eu-strip over strip when elfutils is installed, which is the common
case on RHEL 9. eu-strip removes non-allocated ELF sections, including
the .BTF section that contains BPF Type Format information for kernel
modules. Without .BTF, BPF tools (bpftool, bcc, bpftrace) cannot resolve
kernel types at runtime, and /sys/kernel/btf/<module> entries are not
created when modules are loaded.

Additionally, since commit 8646db238997 ("libbpf,bpf: Share BTF
relocate-related code with kernel"), modules contain a .BTF.base section
that maps distilled type IDs to vmlinux types. If .BTF.base is stripped,
btf_parse_module() falls back to vmlinux BTF directly, causing type ID
mismatches and rejecting the module's BTF entirely.

Fix by passing --keep-section .BTF and --keep-section .BTF.base via
_find_debuginfo_opts, which adds -K .BTF and -K .BTF.base to the
eu-strip/strip command, preserving both sections while allowing normal
debuginfo extraction to proceed.

After this change, all module BTF files are properly generated:

  $ ls /sys/kernel/btf/
  aesni_intel         drm                 i2c_i801            mfd_core
  ahci                drm_client_lib      i2c_mux             net_failover
  backlight           drm_kms_helper      i2c_smbus           pcspkr
  ccp                 drm_shmem_helper    input_leds          qemu_fw_cfg
  dm_log              failover            intel_rapl_common   sch_fq_codel
  dm_mirror           fat                 intel_rapl_msr      serio_raw
  dm_mod              fuse                irqbypass           sunrpc
  dm_region_hash      gf128mul            iTCO_wdt            vfat
  virtio_balloon      virtio_console      virtio_dma_buf      virtio_gpu
  virtio_net          virtio_rng          virtio_blk          vmlinux
  xfs

Suggested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: Alan Maguire <alan.maguire@oracle.com>
Link: https://patch.msgid.link/20260728024950.44946-1-laoar.shao@gmail.com
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

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