compat: fix 4-byte infoleak via uninitialized struct field

Commit 3a4d44b61625 ("ntp: Move adjtimex related compat syscalls to
native counterparts") removed the memset() in compat_get_timex().  Since
then, the compat adjtimex syscall can invoke do_adjtimex() with an
uninitialized ->tai.

If do_adjtimex() doesn't write to ->tai (e.g.  because the arguments are
invalid), compat_put_timex() then copies the uninitialized ->tai field
to userspace.

Fix it by adding the memset() back.

Fixes: 3a4d44b61625 ("ntp: Move adjtimex related compat syscalls to native counterparts")
Signed-off-by: Jann Horn <jannh@google.com>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
int compat_put_timespec64(const struct timespec64 *ts, void __user *uts)
{
	if (COMPAT_USE_64BIT_TIME)
		return copy_to_user(uts, ts, sizeof(*ts)) ? -EFAULT : 0;
	else
		return __compat_put_timespec64(ts, uts);
}

int compat_put_timespec64(const struct timespec64 *ts, void __user *uts)
{
	if (COMPAT_USE_64BIT_TIME)
		return copy_to_user(uts, ts, sizeof(*ts)) ? -EFAULT : 0;
	else
		return __compat_put_timespec64(ts, uts);
}
