eigrpd: Correctly set the mtu for eigrp packets sent

eigrpd: Correctly set the mtu for eigrp packets sent

This version of eigrp pre-calculated the eigrp metric
to be a default of 1500 bytes, but unfortunately it
had entered the byte order wrong.

Modify the code to properly set the byte order
according to the eigrp rfc as well as actually
read in and transmit the mtu of the interface
instead of hard coding it to 1500 bytes.

Fixes: #7986
Signed-off-by: Donald Sharp <sharpd@nvidia.com>