Bluetooth: Use kzalloc instead of kmalloc/memset

Use kzalloc rather than duplicating its implementation, which makes code
simple and easy to understand.

./net/bluetooth/hci_conn.c:2038:6-13: WARNING: kzalloc should be used for cp, instead of kmalloc/memset.

Link: https://bugzilla.openanolis.cn/show_bug.cgi?id=2406
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>