bgpd: avoid allocating very large stack buffer

bgpd: avoid allocating very large stack buffer

As pointed out on code review of BGP extended messages, increasing the
maximum BGP message size has the consequence of growing the dynamically
sized stack buffer up to 650K. While unlikely to exceed modern stack
sizes it is still unreasonably large. Remedy this with a heap buffer.

Signed-off-by: Quentin Young <qlyoung@nvidia.com>