net/mlx5: fix extended statistics

net/mlx5: fix extended statistics

The number of extended statistics counters is queried through ETHTOOL.
ETHTOOL provides a different number when the link is up or down.
Since extended statistics query occurs at device start,
segmentation fault might happen when changing the link state before and
after the device start.

this commit address this issue, and query the number of statistics
before every call to ETHTOOL.

Fixes: a4193ae3bc4f ("net/mlx5: support extended statistics")
Cc: stable@dpdk.org
Signed-off-by: Shahaf Shuler <shahafs@mellanox.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>