selftests/bpf: Cleanup some file descriptors in test_maps

The test_map_rdonly and test_map_wronly tests should close file descriptors
which they open.

Signed-off-by: Anton Protopopov <a.s.protopopov@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200527185700.14658-3-a.s.protopopov@gmail.com


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