bpftool: Ensure bytes_memlock json output is correct

If a BPF map is created over 2^32 the memlock value as displayed in JSON
format will be incorrect. Use atoll instead of atoi so that the correct
number is displayed.

  ```
  $ bpftool map create /sys/fs/bpf/test_bpfmap type hash key 4 \
    value 1024 entries 4194304 name test_bpfmap
  $ bpftool map list
  1: hash  name test_bpfmap  flags 0x0
          key 4B  value 1024B  max_entries 4194304  memlock 4328521728B
  $ sudo bpftool map list -j | jq .[].bytes_memlock
  33554432
  ```

Signed-off-by: Chris J Arges <carges@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/b6601087-0b11-33cc-904a-1133d1500a10@cloudflare.com