bpf: selftests: Add test for different inner map size

This patch tests the inner map size can be different
for reuseport_sockarray but has to be the same for
arraymap.  A new subtest "diff_size" is added for this.

The existing test is moved to a subtest "lookup_update".

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200828011819.1970825-1-kafai@fb.com