RDMA/mlx5: Remove extra unlock on error path

The below commit lifted the locking out of this function but left this
error path unlock behind resulting in unbalanced locking. Remove the
missed unlock too.

Cc: stable@vger.kernel.org
Fixes: 62712228 ("RDMA/mlx5: Add work to remove temporary entries from the cache")
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Michael Guralnik <michaelgur@nvidia.com>
Link: https://lore.kernel.org/r/78090c210c750f47219b95248f9f782f34548bb1.1716900410.git.leon@kernel.orgSigned-off-by: Leon Romanovsky <leon@kernel.org>

drivers/infiniband/hw/mlx5/mr.c

@@ -641,10 +641,8 @@ static int mlx5_cache_ent_insert(struct mlx5_mkey_cache *cache,
 			new = &((*new)->rb_left);
 		if (cmp < 0)
 			new = &((*new)->rb_right);
-		if (cmp == 0) {
-			mutex_unlock(&cache->rb_lock);
+		if (cmp == 0)
 			return -EEXIST;
-		}
 	}
 
 	/* Add new node and rebalance tree. */