test/bpf_cp: use proper SKIP exit code

This test requires two arguments, copying from one to the other. If
two aren't given, use T_EXIT_SKIP to signify that the test was simply
skipped, rather than return success when nothing was done.

Fixes: fd8a6e66c739 ("tests: test io_uring bpf ops")
Signed-off-by: Jens Axboe <axboe@kernel.dk>