sysctl: add proc_dou8vec_minmax()

[ Upstream commit cb944413 ]

Networking has many sysctls that could fit in one u8.

This patch adds proc_dou8vec_minmax() for this purpose.

Note that the .extra1 and .extra2 fields are pointing
to integers, because it makes conversions easier.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: dc5110c2 ("tcp: restrict net.ipv4.tcp_app_win")
Signed-off-by: Sasha Levin <sashal@kernel.org>