net/mlx5: Replace fs_node mutex with reader/writer semaphore

Currently, steering object is protected by mutex lock, replace
the mutex lock with reader/writer semaphore .
In this patch we still use only write semaphore. In downstream
patches we will switch part of the write locks to read locks.

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>