bgpd: The last_reset_cause in the peer structure is too large

bgpd: The last_reset_cause in the peer structure is too large

The last_reset_cause is a plain old BGP_MAX_PACKET_SIZE buffer
that is really enlarging the peer data structure.  Let's just
copy the stream that failed and only allocate how ever much
the packet size actually was.  While it's likely that we have
a reset reason, the packet typically is not going to be 65k
in size.  Let's save space.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>