Merge branch 'xdp_devmap'

David Ahern says:

====================
Implementation of Daniel's proposal for allowing DEVMAP entries to be
a device index, program fd pair.

Programs are run after XDP_REDIRECT and have access to both Rx device
and Tx device.

v4
- moved struct bpf_devmap_val from uapi to devmap.c, named the union
  and dropped the prefix from the elements - Jesper
- fixed 2 bugs in selftests

v3
- renamed struct to bpf_devmap_val
- used offsetofend to check for expected map size, modification of
  Toke's comment
- check for explicit value sizes
- adjusted switch statement in dev_map_run_prog per Andrii's comment
- changed SEC shortcut to xdp_devmap
- changed selftests to use skeleton and new map declaration

v2
- moved dev_map_ext_val definition to uapi to formalize the API for devmap
  extensions; add bpf_ prefix to the prog_fd and prog_id entries
- changed devmap code to handle struct in a way that it can support future
  extensions
- fixed subject in libbpf patch

v1
- fixed prog put on invalid program - Toke
- changed write value from id to fd per Toke's comments about capabilities
- add test cases
====================

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