bpf: Do not dereference user pointer in bpf_test_finish().

Instead, pass the kattr in which has a kernel side copy of this
data structure from userspace already.

Fix based upon a suggestion from Alexei Starovoitov.

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>