net/lwip: fix neighbor cache free function
net/lwip: fix neighbor cache free function

neighbor cache function doesn't clear the lladdr and
probes_sent value and it makes the critical error on
ND6 process.

Change-Id: If74aa05313bb10280076d1eab1ab43a475450a1f
Signed-off-by: jisuu.kim <800dec47d2987cee7cc3d7aee71bc11b56f5042e@samsung.com>

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