iptables: extensions: libxt_MARK: Fix translation of --set-xmark option

iptables: extensions: libxt_MARK: Fix translation of --set-xmark option

Fix translation of MARK target's --set-xmark option.

Before:
  #iptables-translate -t mangle -A PREROUTING -j MARK --set-xmark 0x64/0xaf
  nft add rule ip mangle PREROUTING counter meta mark set mark xor 0x64 and 0xaf

After:

  # iptables-translate -t mangle -A PREROUTING -j MARK --set-xmark 0x64/0xaf
  nft add rule ip mangle PREROUTING counter meta mark set mark and 0xffffff50 \
  xor 0x64

Signed-off-by: Roberto García <rodanber@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>