perf test bpf: Improve message about expected samples

When failing on one of the BPF tests we were just stating:

  BPF filter result incorrect

Add some more info to help figuring out the problem:

 BPF filter result incorrect, expected 56, got 0 samples

This came out while investigating this failure, first seen after
updating the kernel to the 4.15.0-rc6 tag:

  [root@jouet ~]# perf test bpf
  39: BPF filter               :
  39.1: Basic BPF filtering    : FAILED!
  39.2: BPF pinning            : Skip
  39.3: BPF prologue generation: Skip
  39.4: BPF relocation checker : Skip
  [root@jouet ~]#

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-403npu7daupv6b2bmxliv5pk@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>