tcp: convert elligible sysctls to u8

[ Upstream commit 4ecc1baf ]

Many tcp sysctls are either bools or small ints that can fit into u8.

Reducing space taken by sysctls can save few cache line misses
when sending/receiving data while cpu caches are empty,
for example after cpu idle period.

This is hard to measure with typical network performance tests,
but after this patch, struct netns_ipv4 has shrunk
by three cache lines.

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>