printk: use atomic64_t for devkmsg_user.seq

@user->seq is indirectly protected by @logbuf_lock. Once @logbuf_lock
is removed, @user->seq will be no longer safe from an atomicity point
of view.

In preparation for the removal of @logbuf_lock, change it to
atomic64_t to provide this safety.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210303101528.29901-10-john.ogness@linutronix.de