net: add /proc/sys/net/core/txq_reselection_ms control

net: add /proc/sys/net/core/txq_reselection_ms control

Add a new sysctl to control how often a queue reselection
can happen even if a flow has a persistent queue of skbs
in a Qdisc or NIC queue.

A value of zero means the feature is disabled.

Default is 1000 (1 second).

This sysctl is used in the following patch.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20251013152234.842065-4-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>