libbpf: use map_flags when creating maps

This is required to use BPF_MAP_TYPE_LPM_TRIE or any other map type
which requires flags.

Signed-off-by: Craig Gallek <kraig@google.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>