seq_file conversion for proc/atm/devices:
- code inspired from seq_file use in net/core/dev.c;
- atm_dev_lock taken/released in atm_dev_seq_{start/stop};
- add a helper CREATE_SEQ_ENTRY() similar to CREATE_ENTRY() (both are removed
  once conversion is done).
- atm_dev_seq_{start/stop/next} done in net/atm/resource.[ch] to ease
  future handling of atm_devs and locking structure (per Chas suggestion)