bpf: minor cleanups on fd maps and helpers

Some minor cleanups: i) Remove the unlikely() from fd array map lookups
and let the CPU branch predictor do its job, scenarios where there is not
always a map entry are very well valid. ii) Move the attribute type check
in the bpf_perf_event_read() helper a bit earlier so it's consistent wrt
checks with bpf_perf_event_output() helper as well. iii) remove some
comments that are self-documenting in kprobe_prog_is_valid_access() and
therefore make it consistent to tp_prog_is_valid_access() as well.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>