bpftool: Fail map dump for maps that can't be iterated

"bpftool map dump" walks the map with bpf_map_get_next_key() and treats
ENOENT as the end of the map. Map types that can't be iterated fail that
call on the very first key: local storage and ringbuf with ENOTSUPP,
bloom filter and arena with EOPNOTSUPP, queue and stack with EINVAL.
bpftool then ends the walk without a word and prints what it prints for
an empty map, "[]" with BTF or "Found 0 elements" without, and only the
exit status tells that the map was not read. On a socket storage map
that holds storage for a socket:

  # bpftool map dump pinned /sys/fs/bpf/sk_storage
  []

Ask for the first key before printing anything, and when that fails with
anything but ENOENT, fail with the map type and the error instead of a
dump:

  # bpftool map dump pinned /sys/fs/bpf/sk_storage
  Error: can't dump sk_storage map: Operation not supported
  # bpftool -j map dump pinned /sys/fs/bpf/sk_storage
  {"error":"can't dump sk_storage map: Operation not supported"}

An error later in the walk is now reported too, as "bpftool map getnext"
reports it, and the element count is printed only after a complete walk.

Signed-off-by: Maxim Skokov <skokovmaksimevg@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Quentin Monnet <qmo@kernel.org>
Link: https://patch.msgid.link/20261016194825.230433-3-skokovmaksimevg@gmail.com

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