build: pick the BTF kernel by capability, not by directory name

[majianpeng]

GEN-BTF decided which kernel to pull ceph.ko's BTF from with:

kver=$(find /lib/modules -mindepth 1 -maxdepth 1 -type d -printf '%f\n' \
       | sort -V | tail -1)

i.e. "the highest-sorting directory directly under /lib/modules is the newest kernel". That assumes every such directory is named after a kernel release. Distributions are not obliged to comply.

Kylin Linux Advanced Server V11 (Swan25) ships /lib/modules/ksaf: a kernel-release-independent directory holding Kylin's own LSM modules (ksaf_main.ko from ksaf-main-module, kysec_exectl.ko from kysec2-exectl-module). It is deliberately kept outside /lib/modules// so that the dracut module 98ksaf_main can copy one fixed path into the initramfs for every kernel, and the security modules get loaded as early as possible. It contains no ceph.ko.

GNU sort -V ranks a run of letters ABOVE a run of digits, so any name that starts with a letter sorts above every real version number:

$ find /lib/modules -mindepth 1 -maxdepth 1 -type d -printf '%f\n' | sort -V
5.4.241-24.0017.26
6.6.0-32.12.v2505.ky11.x86_64
6.6.0-32.21.v2505.ky11.x86_64
6.6.0-32.22.v2505.ky11.x86_64
6.6.0-32.24.v2505.ky11.x86_64
ksaf

Hence tail -1 returns ksaf, the recipe looks for ceph.ko under /lib/modules/ksaf, finds nothing, and aborts the build:

GEN-BTF  .../src/ceph_btf_local.h
No ceph.ko* found under /lib/modules/ksaf
make: *** [Makefile:164: .../src/ceph_btf_local.h] Error 1

ceph_btf_local.h is a prerequisite of the .bpf.o targets, so the build dies before a single object is compiled, which makes the error look unrelated to kernel discovery. The failure is also conditional: when the generated header is already present, make skips GEN-BTF and the bug stays hidden until a make clean, git clean, or a fresh checkout removes it.

Fix the selection to match on capability instead of on the directory name: a candidate must actually contain a ceph.ko*, which excludes any non-kernel directory no matter what it is called. Prefer the running kernel when it is installed and ships ceph.ko*, because

• tracing attaches to the live kernel, and
• the base BTF fallback /sys/kernel/btf/vmlinux is only valid for the running kernel, so falling back to a different (e.g. newer, not yet rebooted) kernel would otherwise fail later with "No usable vmlinux/base BTF found".

Verified on Kylin V11, kernel 6.6.0-32.24.v2505.ky11.x86_64:

Using installed kernel: 6.6.0-32.24.v2505.ky11.x86_64
Found ceph kernel module: /lib/modules/6.6.0-32.24.v2505.ky11.x86_64/kernel/fs/ceph/ceph.ko.xz
Using base BTF source: /sys/kernel/btf/vmlinux