ip[6]tables-translate: fix test failures when WESP is defined

ip[6]tables-translate: fix test failures when WESP is defined

Protocol number 141 is assigned to a real protocol: Wrapped Encapsulating
Security Payload.  This is listed in Debian's /etc/protocols, which leads to
test failures:

  ./extensions/generic.txlate: Fail
  src: iptables-translate -A FORWARD -p 141
  exp: nft 'add rule ip filter FORWARD ip protocol 141 counter'
  res: nft 'add rule ip filter FORWARD ip protocol wesp counter'

  ./extensions/generic.txlate: Fail
  src: ip6tables-translate -A FORWARD -p 141
  exp: nft 'add rule ip6 filter FORWARD meta l4proto 141 counter'
  res: nft 'add rule ip6 filter FORWARD meta l4proto wesp counter'

  ./extensions/generic.txlate: Fail
  src: iptables-translate -A FORWARD ! -p 141
  exp: nft 'add rule ip filter FORWARD ip protocol != 141 counter'
  res: nft 'add rule ip filter FORWARD ip protocol != wesp counter'

  ./extensions/generic.txlate: Fail
  src: ip6tables-translate -A FORWARD ! -p 141
  exp: nft 'add rule ip6 filter FORWARD meta l4proto != 141 counter'
  res: nft 'add rule ip6 filter FORWARD meta l4proto != wesp counter'

Replace it with 253, which IANA reserves for testing and experimentation.

Fixes: fcaa99ca9e3c ("xtables-translate: Leverage stored protocol names")
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Phil Sutter <phil@nwl.cc>