perf test: Remove bash construct from stat_bpf_counters.sh test

perf test: Remove bash construct from stat_bpf_counters.sh test

Currently the test skips with an error because == only works in bash:

  $ ./perf test 91 -v
  Couldn't bump rlimit(MEMLOCK), failures may take place when creating BPF maps, etc
  91: perf stat --bpf-counters test                                   :
  --- start ---
  test child forked, pid 44586
  ./tests/shell/stat_bpf_counters.sh: 26: [: -v: unexpected operator
  test child finished with -2
  ---- end ----
  perf stat --bpf-counters test: Skip

Changing == to = does the same thing, but doesn't result in an error:

  ./perf test 91 -v
  Couldn't bump rlimit(MEMLOCK), failures may take place when creating BPF maps, etc
  91: perf stat --bpf-counters test                                   :
  --- start ---
  test child forked, pid 45833
  Skipping: --bpf-counters not supported
    Error: unknown option `bpf-counters'
  [...]
  test child finished with -2
  ---- end ----
  perf stat --bpf-counters test: Skip

Signed-off-by: James Clark <james.clark@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: KP Singh <kpsingh@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Sumanth Korikkar <sumanthk@linux.ibm.com>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Cc: Yonghong Song <yhs@fb.com>
Cc: bpf@vger.kernel.org
Cc: netdev@vger.kernel.org
Link: https://lore.kernel.org/r/20211028134828.65774-2-james.clark@arm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>