Fixes #610. Do not use the value of mtu blindly test it is set or use 1500 as default.
Fixes #610. Do not use the value of mtu blindly test it is set or use 1500 as default.

pfsense/pfsense,pfsense/pfsense,NewEraCracker/pfsense,BlackstarGroup/pfsense,ptorsten/pfsense,NewEraCracker/pfsense,ch1c4um/pfsense,dennypage/pfsense,ptorsten/pfsense,dennypage/pfsense,ch1c4um/pfsense,ch1c4um/pfsense,jxmx/pfsense,pfsense/pfsense,brunostein/pfsense,phil-davis/pfsense,brunostein/pfsense,BlackstarGroup/pfsense,brunostein/pfsense,jxmx/pfsense,phil-davis/pfsense,dennypage/pfsense,phil-davis/pfsense,BlackstarGroup/pfsense,ptorsten/pfsense,ch1c4um/pfsense,phil-davis/pfsense,NewEraCracker/pfsense,NOYB/pfsense,NOYB/pfsense,NewEraCracker/pfsense,NOYB/pfsense,BlackstarGroup/pfsense,dennypage/pfsense,pfsense/pfsense,jxmx/pfsense