bpf: guard BTF_ID_FLAGS(bpf_cgroup_read_xattr) with CONFIG_BPF_LSM

Function bpf_cgroup_read_xattr is defined in fs/bpf_fs_kfuncs.c,
which is compiled only when CONFIG_BPF_LSM is set. Add CONFIG_BPF_LSM
check to bpf_cgroup_read_xattr spec in common_btf_ids in
kernel/bpf/helpers.c to avoid build failures for configs w/o
CONFIG_BPF_LSM.

Build failure example:

    BTF     .tmp_vmlinux1.btf.o
  btf_encoder__tag_kfunc: failed to find kfunc 'bpf_cgroup_read_xattr' in BTF
  ...
  WARN: resolve_btfids: unresolved symbol bpf_cgroup_read_xattr
  make[2]: *** [scripts/Makefile.vmlinux:91: vmlinux.unstripped] Error 255

Fixes: 535b070 ("bpf: Introduce bpf_cgroup_read_xattr to read xattr of cgroup's node")
Reported-by: Jake Hillion <jakehillion@meta.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250627175309.2710973-1-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: eddyz87

kernel/bpf/helpers.c

@@ -3779,7 +3779,7 @@ BTF_ID_FLAGS(func, bpf_strspn);
 BTF_ID_FLAGS(func, bpf_strcspn);
 BTF_ID_FLAGS(func, bpf_strstr);
 BTF_ID_FLAGS(func, bpf_strnstr);
-#ifdef CONFIG_CGROUPS
+#if defined(CONFIG_BPF_LSM) && defined(CONFIG_CGROUPS)
 BTF_ID_FLAGS(func, bpf_cgroup_read_xattr, KF_RCU)
 #endif
 BTF_KFUNCS_END(common_btf_ids)