bpf: move user_size out of bpf_test_init

Rely on data_size_in in bpf_test_init routine signature. This is a
preliminary patch to introduce xdp frags selftest

Acked-by: Toke Hoiland-Jorgensen <toke@redhat.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/6b48d38ed3d60240d7d6bb15e6fa7fabfac8dfb2.1642758637.git.lorenzo@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>