bpf: adding support for mapinmap in libbpf

in this patch series i'm adding a helper for libbpf which would allow it to load map-in-map(BPF_MAP_TYPE_ARRAY_OF_MAPS and BPF_MAP_TYPE_HASH_OF_MAPS).
first patch contains new helper + explains proposed workflow second patch contains tests which also could be used as example of usage

v4->v5:
- naming: renamed everything to map_in_map instead of mapinmap
- start to return nonzero val if set_inner_map_fd failed

v3->v4:
- renamed helper to set_inner_map_fd
- now we set this value only if it haven't been set before and only for (array|hash) of maps

v2->v3:
- fixing typo in patch description
- initializing inner_map_fd to -1 by default

v1->v2:
- addressing nits
- removing const identifier from fd in new helper
- starting to check return val for bpf_map_update_elem

Nikita V. Shirokov (2):
bpf: adding support for map in map in libbpf
bpf: adding tests for mapinmap helpber in libbpf

 tools/lib/bpf/libbpf.c                        | 40 ++++++++++--
 tools/lib/bpf/libbpf.h                        |  2 +
 tools/testing/selftests/bpf/Makefile          |  3 +-
 tools/testing/selftests/bpf/test_map_in_map.c | 49 +++++++++++++++
 tools/testing/selftests/bpf/test_maps.c       | 90 +++++++++++++++++++++++++++
 5 files changed, 177 insertions(+), 7 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/test_map_in_map.c

2.15.1