tcp: add a fast path in tcp_delack_timer()

delack timer is not stopped from inet_csk_clear_xmit_timer()
because we do not define INET_CSK_CLEAR_TIMERS.

This is a conscious choice : inet_csk_clear_xmit_timer()
is often called from another cpu. Calling del_timer()
would cause false sharing and lock contention.

This means that very often, tcp_delack_timer() is called
at the timer expiration, while there is no ACK to transmit.

This can be detected very early, avoiding the socket spinlock.

Notes:
- test about tp->compressed_ack is racy,
  but in the unlikely case there is a race, the dedicated
  compressed_ack_timer hrtimer would close it.

- Even if the fast path is not taken, reading
  icsk->icsk_ack.pending and tp->compressed_ack
  before acquiring the socket spinlock reduces
  acquisition time and chances of contention.

Signed-off-by: Eric Dumazet <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

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