u64_stats: Introduce u64_stats_set()

Allow to directly set a u64_stats_t value which is used to provide an init
function which sets it directly to zero intead of memset() the value.

Add u64_stats_set() to the u64_stats API.

[bigeasy: commit message. ]

Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>