ospfd: ospfd core if hello packet exceeds link MTU

ospfd: ospfd core if hello packet exceeds link MTU

Ospfd cored because of an assert when we try to write more than the MTU
size to the ospf packet buffer stream. The problem is - we allocate only MTU
sized buffer. The expectation is that Hello packets are never large
enough to approach MTU. Instead of crashing, this fix discards hello and
logs an error. One should not have so many neighbors behind an
interface.

Ticket: CM-22380
Signed-off-by: Nitin Soni <nsoni@cumulusnetworks.com>
Reviewed-by: CCR-8204