mlx5en: Fix race in mlx5e_ethtool_debug_stats().

Writing to the debug stats variable must be locked,
else serialization will be lost which might cause
various kernel panics due to creating and destroying
sysctls out of order.

Make sure the sysctl context is initialized after freeing
the sysctl nodes, else they can be freed twice.

Submitted by: hselasky@
Approved by: hselasky (mentor)
MFC after: 1 week
Sponsored by: Mellanox Technologies