net/mlx5e: Keep the value for maximum number of channels in-sync

The value for maximum number of channels is first calculated based
on the netdev's profile and current function resources (specifically,
number of MSIX vectors, which depends among other things on the number
of online cores in the system).
This value is then used to calculate the netdev's number of rxqs/txqs.
Once created (by alloc_etherdev_mqs), the number of netdev's rxqs/txqs
is constant and we must not exceed it.

To achieve this, keep the maximum number of channels in sync upon any
netdevice re-attach.

Use mlx5e_get_max_num_channels() for calculating the number of netdev's
rxqs/txqs. After netdev is created, use mlx5e_calc_max_nch() (which
coinsiders core device resources, profile, and netdev) to init or
update priv->max_nch.

Before this patch, the value of priv->max_nch might get out of sync,
mistakenly allowing accesses to out-of-bounds objects, which would
crash the system.

Track the number of channels stats structures used in a separate
field, as they are persistent to suspend/resume operations. All the
collected stats of every channel index that ever existed should be
preserved. They are reset only when struct mlx5e_priv is,
in mlx5e_priv_cleanup(), which is part of the profile changing flow.

There is no point anymore in blocking a profile change due to max_nch
mismatch in mlx5e_netdev_change_profile(). Remove the limitation.

Fixes: a1f240f1

 ("net/mlx5e: Adjust to max number of channles when re-attaching")
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>