evaluate: skip byteorder conversion for selector smaller than 2 bytes

evaluate: skip byteorder conversion for selector smaller than 2 bytes

Add unary expression to trigger byteorder conversion for host byteorder
selectors only if selectors length is larger or equal than 2 bytes.

 # cat test.nft
 table ip x {
        set test {
                type ipv4_addr . ether_addr . inet_proto
                flags interval
        }

        chain y {
                ip saddr . ether saddr . meta l4proto @test counter
        }
 }

 # nft -f test.nft
 ip x y
  [ meta load iiftype => reg 1 ]
  [ cmp eq reg 1 0x00000001 ]
  [ payload load 4b @ network header + 12 => reg 1 ]
  [ payload load 6b @ link header + 6 => reg 9 ]
  [ meta load l4proto => reg 11 ]
  [ byteorder reg 11 = hton(reg 11, 2, 1) ] <--- should not be here
  [ lookup reg 1 set test ]
  [ counter pkts 0 bytes 0 ]

Fixes: 1017d323cafa ("src: support for selectors with different byteorder with interval concatenations")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>