u64_stats: Introduce u64_stats_copy()

u64_stats: Introduce u64_stats_copy()

The following (anti-)pattern was observed in the code tree:

        do {
                start = u64_stats_fetch_begin(&pstats->syncp);
                memcpy(&temp, &pstats->stats, sizeof(temp));
        } while (u64_stats_fetch_retry(&pstats->syncp, start));

On 64bit arches, struct u64_stats_sync is empty and provides no help
against load/store tearing, especially for memcpy(), for which arches may
provide their highly-optimized implements.

In theory the affected code should convert to u64_stats_t, or use
READ_ONCE()/WRITE_ONCE() properly.

However since there are needs to copy chunks of statistics, instead of
writing loops at random places, we provide a safe memcpy() variant for
u64_stats.

Signed-off-by: David Yang <mmyangfl@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260120092137.2161162-2-mmyangfl@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>