net/lwip: update PMTU in destination cache when receiving RA with MTU
net/lwip: update PMTU in destination cache when receiving RA with MTU

This patch adds the code which updates PMTU in destination cache when
Router Advertisement with MTU option is received.

Change-Id: If3b74130f8368a34ab2a73a8430e1c3b9b15a322
Signed-off-by: Wonsang Ryou <bc48fb2bc21aa3a9d3ccd13729d9fc0f5ca913dd@samsung.com>

JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT,JunhwanPark/TizenRT