net/mlx5: Fix not releasing read lock when adding flow rules

If building match list fg fails and we never jumped to
search_again_locked label then the function returned without
unlocking the read lock.

Fixes: bd71b08e ("net/mlx5: Support multiple updates of steering rules in parallel")
Signed-off-by: Roi Dayan <roid@mellanox.com>
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>