netlink_delinearize: clone on netlink_get_register(), release previous on _set()

netlink_delinearize: clone on netlink_get_register(), release previous on _set()

If we add this rule:

  nft add rule filter input meta length 33-55

the listing shows:

  meta length >= 33 meta length <= 754974720

The two meta statements share the same left-hand side, thus, only the
first one is converted from network byte order to host byte order.

Update netlink_get_register() to return a clone so each left-hand side
has its own left-hand side.

Moreover, release the existing register before overriding it with fresh
expressions in netlink_set_register().

Thefore, if you manipulate a register from any of the existing parse
functions, you have to re-set it again to place fresh modified clone.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>