selftest/bpf: Fix kprobe_multi bench test

With [1] the available_filter_functions file contains records
starting with __ftrace_invalid_address___ and marking disabled
entries.

We need to filter them out for the bench test to pass only
resolvable symbols to kernel.

[1] commit b39181f7 ("ftrace: Add FTRACE_MCOUNT_MAX_OFFSET to avoid adding weak function")

Fixes: b39181f7

 ("ftrace: Add FTRACE_MCOUNT_MAX_OFFSET to avoid adding weak function")
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20220615112118.497303-5-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>