rtnl-addr: Inherit prefix length to nl_addr objs in rtnl_addr_set_prefixlen()

Previously if using rtnl_addr_set_prefixlen() the new prefix length
was not forwarded to the corresponding 'struct nl_addr' objects
associated with address already and thus the comparison function
would fail.

This patch also clears the internal ADDR_ATTR_PREFIXLEN flag if
the prefix length has been reset.

Signed-off-by: Thomas Graf <tgraf@suug.ch>