net/mlx5e: Introduce the mlx5e_flush_rq function

Add a function to flush an RQ: clean up descriptors, release pages and
reset the RQ. This procedure is used by the recovery flow, and it will
also be used in a following commit to free some memory when switching a
channel to the XSK mode.

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>