[IPV6] ADDRCONF: Check range of prefix length

As of now, the prefix length is not vaildated when adding or deleting
addresses. The value is passed directly into the inet6_ifaddr structure
and later passed on to memcmp() as length indicator which relies on
the value never to exceed 128 (bits).

Due to the missing check, the currently code allows for any 8 bit
value to be passed on as prefix length while using the netlink
interface, and any 32 bit value while using the ioctl interface.

[Use unsigned int instead to generate better code - yoshfuji]

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>