time/jiffies: Saturate in mult_hz() instead of wrapping

mult_hz() converts a user-supplied seconds value to jiffies for
proc_dointvec_jiffies().  proc_int_u2k_conv_uop() rejects a result above
INT_MAX, but it inspects the product, so a product that wraps arrives as a
small value and is stored.

The input has to exceed ULONG_MAX / HZ for the product to wrap, so the
value below is specific to CONFIG_HZ=1000:

	# echo 18446744073709552 > /proc/sys/net/ipv4/tcp_keepalive_time
	# cat /proc/sys/net/ipv4/tcp_keepalive_time
	0

18446744073709551, one less, is correctly rejected.  Dozens of sysctls
use proc_dointvec_jiffies(), among them tcp_keepalive_time,
tcp_fin_timeout and the conntrack timeouts.

Bound the input in the shape clock_t_to_jiffies() already uses and leave
the INT_MAX policy to the caller.  The bound was open-coded as
"*lvalp > INT_MAX / HZ" until commit 2dc164a48e6f ("sysctl: Create
converter functions with two new macros").

Fixes: 2dc164a48e6f ("sysctl: Create converter functions with two new macros")
Cc: stable@vger.kernel.org
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
Signed-off-by: Joel Granados <joel.granados@kernel.org>