bpf: test_maps: cleanup sockmaps when test ends

Bug: BPF programs and maps related to sockmaps test exist
in memory even after test_maps ends.

This patch fixes it as a short term workaround (sockmap
kernel side needs real fixing) by empyting sockmaps when
test ends.

Fixes: 6f6d33f3

 ("bpf: selftests add sockmap tests")
Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
[ daniel: Note on workaround. ]
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>