net/mlx5: Don't unlock fte while still using it

When adding a new rule to an fte, we need to hold the fte lock
until we add that rule to the fte and increase the fte ref count.

Fixes: 0c56b975 ("net/mlx5_core: Introduce flow steering API")
Signed-off-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>