net/mlx5: optimize critical section in device free

net/mlx5: optimize critical section in device free

When PMD releases shared IB device context, It locks the
mlx5_ibv_list_mutex lock throughout the function so that it does not
happen while removing a device from the list, another process will try
to insert another device into it.
On the other hand, having removed the device from the list even if it
has not yet released all of its resources, it should not care about
other processes and can release the lock.

However, the PMD does not release the lock even though it can, and
performs a number of operations, some of which include sleep and may be
long.
To improve this, shorten the lock time to the minimum necessary.

Signed-off-by: Michael Baum <michaelba@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>