selftests: forwarding: Disable learning before link up

Disable learning before bringing the bridge port up in order to avoid
the FDB being populated and the test failing.

Before:

 # ./bridge_locked_port.sh
 RTNETLINK answers: File exists
 TEST: Locked port ipv4                                              [FAIL]
         Ping worked after locking port, but before adding FDB entry
 TEST: Locked port ipv6                                              [ OK ]
 TEST: Locked port vlan                                              [ OK ]

After:

 # ./bridge_locked_port.sh
 TEST: Locked port ipv4                                              [ OK ]
 TEST: Locked port ipv6                                              [ OK ]
 TEST: Locked port vlan                                              [ OK ]

Fixes: b2b681a4 ("selftests: forwarding: tests of locked port feature")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>