MFC r347262:
Split mlx5e_update_stats_work() in mlx5en(4).

Split the function into the mlx5e_update_stats_locked() core and make
mlx5e_update_stats_work() call the _locked helper, similar to many other
places in the kernel. This improves the code structure, making the
locking clean.

Submitted by: kib@
Sponsored by: Mellanox Technologies