net/mlx5: DR, Add missing mutex init/destroy in pattern manager

Add missing mutex init/destroy as caught by the lock's debug warning:
 DEBUG_LOCKS_WARN_ON(lock->magic != lock)

Fixes: da5d0027

 ("net/mlx5: DR, Add cache for modify header pattern")
Signed-off-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>