perf test: Fix 'perf test BPF' when it fails to find a suitable vmlinux

perf test: Fix 'perf test BPF' when it fails to find a suitable vmlinux

Two bugs in 'perf test BPF' are found when testing BPF prologue without
vmlinux:

 # mv /lib/modules/4.3.0-rc4+/build/vmlinux{,.bak}
 # ./perf test BPF
 37: Test BPF filter             :Failed to find the path for kernel: No such file or directory
 Ok

Test BPF should fail in this case.

After this patch:

 # ./perf test BPF
 37: Test BPF filter             :Failed to find the path for kernel: No such file or directory
  FAILED!
 # mv /lib/modules/4.3.0-rc4+/build/vmlinux{.bak,}
 # ./perf test BPF
 37: Test BPF filter             : Ok

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1447749170-175898-2-git-send-email-wangnan0@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>