selftests: fib_nexthops: Add test cases for FDB status change

selftests: fib_nexthops: Add test cases for FDB status change

Add the following test cases for both IPv4 and IPv6:

* Can change from FDB nexthop to non-FDB nexthop and vice versa.
* Can change FDB nexthop address while in a group.
* Cannot change from FDB nexthop to non-FDB nexthop and vice versa while
  in a group.

Output without "nexthop: Forbid FDB status change while nexthop is in a
group":

 # ./fib_nexthops.sh -t "ipv6_fdb_grp_fcnal ipv4_fdb_grp_fcnal"

 IPv6 fdb groups functional
 --------------------------
 [...]
 TEST: Replace FDB nexthop to non-FDB nexthop                        [ OK ]
 TEST: Replace non-FDB nexthop to FDB nexthop                        [ OK ]
 TEST: Replace FDB nexthop address while in a group                  [ OK ]
 TEST: Replace FDB nexthop to non-FDB nexthop while in a group       [FAIL]
 TEST: Replace non-FDB nexthop to FDB nexthop while in a group       [FAIL]
 [...]

 IPv4 fdb groups functional
 --------------------------
 [...]
 TEST: Replace FDB nexthop to non-FDB nexthop                        [ OK ]
 TEST: Replace non-FDB nexthop to FDB nexthop                        [ OK ]
 TEST: Replace FDB nexthop address while in a group                  [ OK ]
 TEST: Replace FDB nexthop to non-FDB nexthop while in a group       [FAIL]
 TEST: Replace non-FDB nexthop to FDB nexthop while in a group       [FAIL]
 [...]

 Tests passed:  36
 Tests failed:   4
 Tests skipped:  0

Output with "nexthop: Forbid FDB status change while nexthop is in a
group":

 # ./fib_nexthops.sh -t "ipv6_fdb_grp_fcnal ipv4_fdb_grp_fcnal"

 IPv6 fdb groups functional
 --------------------------
 [...]
 TEST: Replace FDB nexthop to non-FDB nexthop                        [ OK ]
 TEST: Replace non-FDB nexthop to FDB nexthop                        [ OK ]
 TEST: Replace FDB nexthop address while in a group                  [ OK ]
 TEST: Replace FDB nexthop to non-FDB nexthop while in a group       [ OK ]
 TEST: Replace non-FDB nexthop to FDB nexthop while in a group       [ OK ]
 [...]

 IPv4 fdb groups functional
 --------------------------
 [...]
 TEST: Replace FDB nexthop to non-FDB nexthop                        [ OK ]
 TEST: Replace non-FDB nexthop to FDB nexthop                        [ OK ]
 TEST: Replace FDB nexthop address while in a group                  [ OK ]
 TEST: Replace FDB nexthop to non-FDB nexthop while in a group       [ OK ]
 TEST: Replace non-FDB nexthop to FDB nexthop while in a group       [ OK ]
 [...]

 Tests passed:  40
 Tests failed:   0
 Tests skipped:  0

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20250921150824.149157-4-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>