This patch adds pretty print support to the basic arraymap.
Support for other bpf maps can be added later.

This patch adds new attrs to the BPF_MAP_CREATE command to allow
specifying the btf_fd, btf_key_id and btf_value_id.  The
BPF_MAP_CREATE can then associate the btf to the map if
the creating map supports BTF.

A BTF supported map needs to implement two new map ops,
map_seq_show_elem() and map_check_btf().  This patch has
implemented these new map ops for the basic arraymap.

It also adds file_operations, bpffs_map_fops, to the pinned
map such that the pinned map can be opened and read.
After that, the user has an intuitive way to do
"cat bpffs/pathto/a-pinned-map" instead of getting
an error.

bpffs_map_fops should not be extended further to support
other operations.  Other operations (e.g. write/key-lookup...)
should be realized by the userspace tools (e.g. bpftool) through
the BPF_OBJ_GET_INFO_BY_FD, map's lookup/update interface...etc.
Follow up patches will allow the userspace to obtain
the BTF from a map-fd.

Here is a sample output when reading a pinned arraymap
with the following map's value:

struct map_value {
	int count_a;
	int count_b;
};

cat /sys/fs/bpf/pinned_array_map:

0: {1,2}
1: {3,4}
2: {5,6}
...
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Alexei Starovoitov <ast@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>