inet_diag: report delayed ack timer information

inet_diag: report delayed ack timer information

inet_sk_diag_fill() populates r->idiag_timer with the following
precedence order:

1 - Retransmit timer.
4 - Probe0 timer.
2 - Keepalive timer.

This patch adds a new value, last in the list, if other timers
are not active.

5 - Delayed ACK timer.

A corresponding iproute2 patch will follow to replace "unknown"
with "delack":

ESTAB 10     0   [2002:a05:6830:1f86::]:12875 [2002:a05:6830:1f85::]:50438

    timer:(unknown,003ms,0) ino:152178 sk:3004 cgroup:unreachable:189 <->

    skmem:(r1344,rb12780520,t0,tb262144,f2752,w0,o250,bl0,d0) ts usec_ts
    ...

Also add the following enum in uapi/linux/inet_diag.h
as suggested by David Ahern.

enum {
	IDIAG_TIMER_OFF,
	IDIAG_TIMER_ON,
	IDIAG_TIMER_KEEPALIVE,
	IDIAG_TIMER_TIMEWAIT,
	IDIAG_TIMER_PROBE0,
	IDIAG_TIMER_DELACK,
};

Neal Cardwell suggested to test for ICSK_ACK_TIMER:
inet_csk_clear_xmit_timer() does not call sk_stop_timer()
because INET_CSK_CLEAR_TIMERS is unset.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260305114829.2163276-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>