samples/bpf: add map_lookup microbenchmark

$ map_perf_test 128
speed of HASH bpf_map_lookup_elem() in lookups per second
	w/o JIT		w/JIT
before	46M		58M
after	42M		74M

perf report
before:
    54.23%  map_perf_test  [kernel.kallsyms]  [k] __htab_map_lookup_elem
    14.24%  map_perf_test  [kernel.kallsyms]  [k] lookup_elem_raw
     8.84%  map_perf_test  [kernel.kallsyms]  [k] htab_map_lookup_elem
     5.93%  map_perf_test  [kernel.kallsyms]  [k] bpf_map_lookup_elem
     2.30%  map_perf_test  [kernel.kallsyms]  [k] bpf_prog_da4fc6a3f41761a2
     1.49%  map_perf_test  [kernel.kallsyms]  [k] kprobe_ftrace_handler

after:
    60.03%  map_perf_test  [kernel.kallsyms]  [k] __htab_map_lookup_elem
    18.07%  map_perf_test  [kernel.kallsyms]  [k] lookup_elem_raw
     2.91%  map_perf_test  [kernel.kallsyms]  [k] bpf_prog_da4fc6a3f41761a2
     1.94%  map_perf_test  [kernel.kallsyms]  [k] _einittext
     1.90%  map_perf_test  [kernel.kallsyms]  [k] __audit_syscall_exit
     1.72%  map_perf_test  [kernel.kallsyms]  [k] kprobe_ftrace_handler

Notice that bpf_map_lookup_elem() and htab_map_lookup_elem() are trivial
functions, yet they take sizeable amount of cpu time.
htab_map_gen_lookup() removes bpf_map_lookup_elem() and converts
htab_map_lookup_elem() into three BPF insns which causing cpu time
for bpf_prog_da4fc6a3f41761a2() slightly increase.

$ map_perf_test 256
speed of ARRAY bpf_map_lookup_elem() in lookups per second
	w/o JIT		w/JIT
before	97M		174M
after	64M		280M

before:
    37.33%  map_perf_test  [kernel.kallsyms]  [k] array_map_lookup_elem
    13.95%  map_perf_test  [kernel.kallsyms]  [k] bpf_map_lookup_elem
     6.54%  map_perf_test  [kernel.kallsyms]  [k] bpf_prog_da4fc6a3f41761a2
     4.57%  map_perf_test  [kernel.kallsyms]  [k] kprobe_ftrace_handler

after:
    32.86%  map_perf_test  [kernel.kallsyms]  [k] bpf_prog_da4fc6a3f41761a2
     6.54%  map_perf_test  [kernel.kallsyms]  [k] kprobe_ftrace_handler

array_map_gen_lookup() removes calls to array_map_lookup_elem()
and bpf_map_lookup_elem() and replaces them with 7 bpf insns.

The performance without JIT is slower, since executing extra insns
in the interpreter is slower than running native C code,
but with JIT the performance gains are obvious,
since native C->x86 code is replaced with fewer bpf->x86 instructions.

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