net/mlx5e: Fix use-after-free in mlx5e_stats_grp_sw_update_stats

We need to sync page pool stats only for active channels. Reading ethtool
stats on a down netdev or a netdev with modified number of channels will
result in a user-after-free, trying to access page pools that are freed
already.

BUG: KASAN: use-after-free in mlx5e_stats_grp_sw_update_stats+0x465/0xf80
Read of size 8 at addr ffff888004835e40 by task ethtool/720

Fixes: cc10e84b

 ("mlx5: add support for page_pool_get_stats")
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Reported-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Joe Damato <jdamato@fastly.com>
Link: https://lore.kernel.org/r/20220312005353.786255-1-saeed@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>