bpf: Add bpf_ksym_find function

bpf: Add bpf_ksym_find function

Adding bpf_ksym_find function that is used bpf bpf address
lookup functions:
  __bpf_address_lookup
  is_bpf_text_address

while keeping bpf_prog_kallsyms_find to be used only for lookup
of bpf_prog objects (will happen in following changes).

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200312195610.346362-8-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>