Adding load_kallsyms_refresh function to re-read symbols from
/proc/kallsyms file.

This will be needed to get proper functions addresses from
bpf_testmod.ko module, which is loaded/unloaded several times
during the tests run, so symbols might be already old when
we need to use them.
Acked-by: NSong Liu <song@kernel.org>
Signed-off-by: NJiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20221025134148.3300700-6-jolsa@kernel.orgSigned-off-by: NAlexei Starovoitov <ast@kernel.org>