ipv6: use ND_REACHABLE_TIME and ND_RETRANS_TIMER instead of magic number

ND_REACHABLE_TIME and ND_RETRANS_TIMER have defined
since v2.6.12-rc2, but never been used.
So use them instead of magic number.

This patch also changes original code style to read comfortably .

Thank YOSHIFUJI Hideaki for pointing it out.

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>