bpf: implement decoding of BPF_BTF_LOAD command

bpf: implement decoding of BPF_BTF_LOAD command

BPF_BTF_LOAD command was introduced by Linux commit
v4.18-rc1~114^2~417^2~1^2~5.

* bpf_attr.h (struct BPF_BTF_LOAD_struct): New type.
(BPF_BTF_LOAD_struct_size, expected_BPF_BTF_LOAD_struct_size): New
macros.
* bpf.c (BEGIN_BPF_CMD_DECODER(BPF_BTF_LOAD)): New bpf command decoder.
(SYS_FUNC(bpf)) <bpf_cmd_decoders[]>: Add BPF_CMD_ENTRY(BPF_BTF_LOAD).
* tests/bpf.c (union bpf_attr_data): Add
BPF_ATTR_DATA_FIELD(BPF_BTF_LOAD).
(init_BPF_BTF_LOAD_attr): New function.
(BPF_BTF_LOAD_checks): New checks array.
(main) <checks>: Add CHK(BPF_BTF_LOAD).