meta: stash context statement length when generating payload/meta dependency

meta: stash context statement length when generating payload/meta dependency

... meta mark set ip dscp

generates an implicit dependency from the inet family to match on meta
nfproto ip.

The length of this implicit expression is incorrectly adjusted to the
statement length, ie. relational to compare meta nfproto takes 4 bytes
instead of 1 byte. The evaluation of 'ip dscp' under the meta mark
statement triggers this implicit dependency which should not consider
the context statement length since it is added before the statement
itself.

This problem shows when listing the ruleset, since netlink_parse_cmp()
where left->len < right->len, hence handling the implicit dependency as
a concatenation, but it is actually a bug in the evaluation step that
leads to incorrect bytecode.

Fixes: 3c64ea7995cb ("evaluate: honor statement length in integer evaluation")
Fixes: edecd58755a8 ("evaluate: support shifts larger than the width of the left operand")
Tested-by: Brian Davidson <davidson.brian@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>