evaluate: reset statement length context only for set mappings

evaluate: reset statement length context only for set mappings

map expression (which is used a key to look up for the mapping) needs to
consider the statement length context, otherwise incorrect bytecode is
generated when {ct,meta} statement is generated.

 # nft -f - <<EOF
 add table ip6 t
 add chain ip6 t c
 add map ip6 t mapv6 { typeof ip6 dscp : meta mark; }
 EOF

 # nft -d netlink add rule ip6 t c meta mark set ip6 dscp map @mapv6
 ip6 t c
   [ payload load 2b @ network header + 0 => reg 1 ]
   [ bitwise reg 1 = ( reg 1 & 0x0000c00f ) ^ 0x00000000 ]
   ... missing byteorder conversion here before shift ...
   [ bitwise reg 1 = ( reg 1 >> 0x00000006 ) ]
   [ lookup reg 1 set mapv6 dreg 1 ]
   [ meta set mark with reg 1 ]

Reset statement length context only for the mapping side for the
elements in the set.

Fixes: edecd58755a8 ("evaluate: support shifts larger than the width of the left operand")
Reported-by: Brian Davidson <davidson.brian@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>