Merge branch 'bpf_enable_stats'

Song Liu says:

====================
run_time_ns is a useful stats for BPF programs. However, it is gated by
sysctl kernel.bpf_stats_enabled. When multiple user space tools are
toggling kernl.bpf_stats_enabled at the same time, they may confuse each
other.

Solve this problem with a new BPF command BPF_ENABLE_STATS.

Changes v8 => v9:
  1. Clean up in selftest (Andrii).
  2. Not using static variable in test program (Andrii).

Changes v7 => v8:
  1. Change name BPF_STATS_RUNTIME_CNT => BPF_STATS_RUN_TIME (Alexei).
  2. Add CHECK_ATTR to bpf_enable_stats() (Alexei).
  3. Rebase (Andrii).
  4. Simplfy the selftest (Alexei).

Changes v6 => v7:
  1. Add test to verify run_cnt matches count measured by the program.

Changes v5 => v6:
  1. Simplify test program (Yonghong).
  2. Rebase (with some conflicts).

Changes v4 => v5:
  1. Use memset to zero bpf_attr in bpf_enable_stats() (Andrii).

Changes v3 => v4:
  1. Add libbpf support and selftest;
  2. Avoid cleaning trailing space.

Changes v2 => v3:
  1. Rename the command to BPF_ENABLE_STATS, and make it extendible.
  2. fix commit log;
  3. remove unnecessary headers.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>