bpf: make preloaded map iterators to display map elements count

Add another column to the /sys/fs/bpf/maps.debug iterator to display
cur_entries, the current number of entries in the map as is returned
by the bpf_map_sum_elem_count kfunc. Also fix formatting.

Example:

    # cat /sys/fs/bpf/maps.debug
      id name             max_entries  cur_entries
       2 iterator.rodata            1            0
     125 cilium_auth_map       524288          666
     126 cilium_runtime_          256            0
     127 cilium_signals            32            0
     128 cilium_node_map        16384         1344
     129 cilium_events             32            0
     ...

Signed-off-by: Anton Protopopov <aspsk@isovalent.com>
Link: https://lore.kernel.org/r/20230706133932.45883-5-aspsk@isovalent.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>