Bluetooth: RFCOMM: fix ODEBUG bug in rfcomm_dev_ioctl

Needn't call 'rfcomm_dlc_put' here, because 'rfcomm_dlc_exists' didn't
increase dlc->refcnt.

Reported-by:  <syzbot+4496e82090657320efc6@syzkaller.appspotmail.com>
Signed-off-by: Qiujun Huang <hqjagain@gmail.com>
Suggested-by: Hillf Danton <hdanton@sina.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>