net/mlx5: Bridge, take rtnl lock in init error handler

The mlx5_esw_bridge_cleanup() is expected to be called with rtnl lock
taken, which is true for mlx5e_rep_bridge_cleanup() function but not for
error handling code in mlx5e_rep_bridge_init(). Add missing rtnl
lock/unlock calls and extend both mlx5_esw_bridge_cleanup() and its dual
function mlx5_esw_bridge_init() with ASSERT_RTNL() to verify the invariant
from now on.

Fixes: 7cd6a54a ("net/mlx5: Bridge, handle FDB events")
Fixes: 19e9bfa0 ("net/mlx5: Bridge, add offload infrastructure")
Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>