ipv6: Expose reachable and retrans timer values as msecs

Expose reachable and retrans timer values in msecs instead of jiffies.
Both timer values are already exposed as msecs in the neighbour table
netlink interface.

The creation timestamp format with increased precision is kept but
cleaned up.

Signed-off-by: Thomas Graf <tgraf@infradead.org>
Cc: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>