samples/bpf: fix a couple of style issues in bpf_load

This commit fixes a few style problems in samples/bpf/bpf_load.c:

 - Magic string use of 'DEBUGFS'
 - Useless zero initialization of a global variable
 - Minor style fix with whitespace

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>