bgpd: Optimize memory usage for bgp_notify struct

bgpd: Optimize memory usage for bgp_notify struct

```
struct bgp_notify {
	uint8_t                    code;                 /*     0     1 */
	uint8_t                    subcode;              /*     1     1 */
	bgp_size_t                 length;               /*     2     2 */
	_Bool                      hard_reset;           /*     4     1 */

	/* XXX 3 bytes hole, try to pack */

	char *                     data;                 /*     8     8 */
	uint8_t *                  raw_data;             /*    16     8 */

	/* size: 24, cachelines: 1, members: 6 */
	/* sum members: 21, holes: 1, sum holes: 3 */
	/* last cacheline: 24 bytes */
};   /* saved 16 bytes! */
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>