netlink: fix prefix expression handling

netlink: fix prefix expression handling

The prefix expression handling is full of bugs:

- netlink_gen_data() is used to construct the prefix mask from the full
  prefix expression. This is both conceptually wrong, the prefix expression
  is *not* data, and buggy, it only assumes network masks and thus only
  handles big endian types.

- Prefix expression reconstruction doesn't check whether the mask is a
  valid prefix and reconstructs crap otherwise. It doesn't reconstruct
  prefixes for anything but network addresses. On top of that its
  needlessly complicated, using the mpz values directly its a simple
  matter of finding the sequence of 1's that extend up to the full width.

- Unnecessary cloning of expressions where a simple refcount increase would
  suffice.

Rewrite that code properly.

Signed-off-by: Patrick McHardy <kaber@trash.net>