selftests: fib_rule_tests: Add DSCP mask match tests

Add tests for FIB rules that match on DSCP with a mask. Test both good
and bad flows and both the input and output paths.

 # ./fib_rule_tests.sh
 IPv6 FIB rule tests
 [...]
    TEST: rule6 check: dscp redirect to table                           [ OK ]
    TEST: rule6 check: dscp no redirect to table                        [ OK ]
    TEST: rule6 del by pref: dscp redirect to table                     [ OK ]
    TEST: rule6 check: iif dscp redirect to table                       [ OK ]
    TEST: rule6 check: iif dscp no redirect to table                    [ OK ]
    TEST: rule6 del by pref: iif dscp redirect to table                 [ OK ]
    TEST: rule6 check: dscp masked redirect to table                    [ OK ]
    TEST: rule6 check: dscp masked no redirect to table                 [ OK ]
    TEST: rule6 del by pref: dscp masked redirect to table              [ OK ]
    TEST: rule6 check: iif dscp masked redirect to table                [ OK ]
    TEST: rule6 check: iif dscp masked no redirect to table             [ OK ]
    TEST: rule6 del by pref: iif dscp masked redirect to table          [ OK ]
 [...]

 Tests passed: 316
 Tests failed:   0

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Link: https://patch.msgid.link/20250220080525.831924-7-idosch@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>