net/mlx5: Change lag mutex lock to spin lock

The lag lock could be a spin lock, the critical section is short
and there is no need that the thread will sleep.
Change the lock that protects the LAG structure from mutex
to spin lock. It is required for next patch that need to
access this structure from context that we can't sleep.
In addition there is no need to hold this lock when query the
congestion counters.

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>