net/mlx5: DR, Lower sync threshold for ICM hot memory

Instead of hiding the math in the code, define a value that sets the
fraction of allowed hot memory of ICM pool.
Set the threshold for sync of ICM hot chunks to 1/4 of the pool
instead of 1/2 of the pool. Although we will have more syncs, each
sync will be shorter and will help with insertion rate stability.

Signed-off-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
Reviewed-by: Alex Vesker <valex@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>