selftests: forwarding: skip if kernel not support setting bridge fdb learning limit

If the testing kernel doesn't support setting fdb_max_learned or show
fdb_n_learned, just skip it. Or we will get errors like

./bridge_fdb_learning_limit.sh: line 218: [: null: integer expression expected
./bridge_fdb_learning_limit.sh: line 225: [: null: integer expression expected

Fixes: 6f840903

 ("selftests: forwarding: bridge_fdb_learning_limit: Add a new selftest")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Johannes Nixdorf <jnixdorf-oss@avm.de>
Signed-off-by: David S. Miller <davem@davemloft.net>