selftests: net: Add missing matchall classifier

selftests: net: Add missing matchall classifier

One of the test cases in the test_bridge_backup_port.sh selftest relies
on a matchall classifier to drop unrelated traffic so that the Tx drop
counter on the VXLAN device will only be incremented as a result of
traffic generated by the test.

However, the configuration option for the matchall classifier is
missing from the configuration file which might explain the failures we
see in the netdev CI [1].

Fix by adding CONFIG_NET_CLS_MATCHALL to the configuration file.

[1]
 # Backup nexthop ID - invalid IDs
 # -------------------------------
 [...]
 # TEST: Forwarding out of vx0                                         [ OK ]
 # TEST: No forwarding using backup nexthop ID                         [ OK ]
 # TEST: Tx drop increased                                             [FAIL]
 # TEST: IPv6 address family nexthop as backup nexthop                 [ OK ]
 # TEST: No forwarding out of swp1                                     [ OK ]
 # TEST: Forwarding out of vx0                                         [ OK ]
 # TEST: No forwarding using backup nexthop ID                         [ OK ]
 # TEST: Tx drop increased                                             [FAIL]
 [...]

Fixes: b408453053fb ("selftests: net: Add bridge backup port and backup nexthop ID test")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://lore.kernel.org/r/20240129123703.1857843-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>