bpf: Switch to offsetofend in BPF_PROG_TEST_RUN

Switch existing pattern of "offsetof(..., member) + FIELD_SIZEOF(...,
member)' to "offsetofend(..., member)" which does exactly what
we need without all the copy-paste.

Suggested-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20191210191933.105321-1-sdf@google.com