net: move sysctl_mem_pcpu_rsv to net_hotdata

sysctl_mem_pcpu_rsv is used in TCP fast path,
move it to net_hodata for better cache locality.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240429134025.1233626-6-edumazet@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>