bpf: enhance decoding of BPF_MAP_LOOKUP_ELEM and BPF_MAP_GET_NEXT_KEY

bpf: enhance decoding of BPF_MAP_LOOKUP_ELEM and BPF_MAP_GET_NEXT_KEY

Print union bpf_attr.value field of BPF_MAP_LOOKUP_ELEM command
and union bpf_attr.next_key field of BPF_MAP_GET_NEXT_KEY command
on entering syscall.  These fields are addresses specified to the
kernel from userspace.  The amount of data written by the kernel
to these addresses is specified at the map creation time
by BPF_MAP_CREATE command and is not available at this point.

* bpf.c (decode_BPF_MAP_LOOKUP_ELEM): Print union bpf_attr.value
on entering syscall.
(decode_BPF_MAP_GET_NEXT_KEY): Print union bpf_attr.next_key
on entering syscall.
(bpf_map_io): Remove.
* tests/bpf.c (print_BPF_MAP_DELETE_ELEM_first,
print_BPF_MAP_DELETE_ELEM_attr, print_BPF_MAP_GET_NEXT_KEY_first,
print_BPF_MAP_GET_NEXT_KEY_attr): Replace macro redirects with
new functions.
(print_BPF_MAP_LOOKUP_ELEM_first, print_BPF_MAP_LOOKUP_ELEM_attr,