bpf: change handling of big and unaccessible data to match the kernel

bpf: change handling of big and unaccessible data to match the kernel

When the size argument exceeds PAGE_SIZE, the kernel fails with E2BIG
without parsing union bpf_attr.
When the whole chunk of memory specified by addr and size arguments is
not readable, the kernel fails with EFAULT.

* bpf.c (DECL_BPF_CMD_DECODER) <bpf_cmd_decoder>: Add const qualifier
to size argument, add data argument.
(decode_BPF_MAP_CREATE, decode_BPF_MAP_UPDATE_ELEM,
decode_BPF_MAP_DELETE_ELEM, bpf_map_io, decode_BPF_PROG_LOAD,
bpf_obj_manage, bpf_prog_attach_detach): Move size argument check and
memory fetching ...
(SYS_FUNC(bpf)) ... here, add PAGE_SIZE check, pass fetched memory
to command-specific parsers.