selftests/bpf: Refactor bpf_get_ksyms() trace helper

selftests/bpf: Refactor bpf_get_ksyms() trace helper

ASAN reported a memory leak in bpf_get_ksyms(): it allocates a struct
ksyms internally and never frees it.

Move struct ksyms to trace_helpers.h and return it from the
bpf_get_ksyms(), giving ownership to the caller. Add filtered_syms and
filtered_cnt fields to the ksyms to hold the filtered array of
symbols, previously returned by bpf_get_ksyms().

Fixup the call sites: kprobe_multi_test and bench_trigger.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260223190736.649171-10-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>