u64_stats: fix u64_stats_init() for lockdep when used repeatedly in one file

u64_stats: fix u64_stats_init() for lockdep when used repeatedly in one file

Fix bogus lockdep warnings if multiple u64_stats_sync variables are
initialized in the same file.

With CONFIG_LOCKDEP, seqcount_init() is a macro which declares:

	static struct lock_class_key __key;

Since u64_stats_init() is a function (albeit an inline one), all calls
within the same file end up using the same instance, effectively treating
them all as a single lock-class.

Fixes: 9464ca650008 ("net: make u64_stats_init() a function")
Closes: https://lore.kernel.org/netdev/ea1567d9-ce66-45e6-8168-ac40a47d1821@roeck-us.net/
Signed-off-by: Petr Tesarik <petr@tesarici.cz>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240404075740.30682-1-petr@tesarici.cz
Signed-off-by: Jakub Kicinski <kuba@kernel.org>