selftest/bpf: extend the offload test with map checks

selftest/bpf: extend the offload test with map checks

Check map device information is reported correctly, and perform
basic map operations.  Check device destruction gets rid of the
maps and map allocation failure path by telling netdevsim to
reject map offload via DebugFS.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>