samples/bpf: add error checking for perf ioctl calls in bpf loader

load_bpf_file() should fail if ioctl with command
PERF_EVENT_IOC_ENABLE and PERF_EVENT_IOC_SET_BPF fails.
When they do fail, proper error messages are printed.

With this change, the below "syscall_tp" run shows that
the maximum number of bpf progs attaching to the same
perf tracepoint is indeed enforced.
  $ ./syscall_tp -i 64
  prog #0: map ids 4 5
  ...
  prog #63: map ids 382 383
  $ ./syscall_tp -i 65
  prog #0: map ids 4 5
  ...
  prog #64: map ids 388 389
  ioctl PERF_EVENT_IOC_SET_BPF failed err Argument list too long

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>