selftests: xsk: Simplify cleanup of ifobjects

Simpify the cleanup of ifobjects right before the program exits by
introducing functions for creating and destroying these objects.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210825093722.10219-13-magnus.karlsson@gmail.com