tcp: improve latencies of timer triggered events

Modern TCP stack highly depends on tcp_write_timer() having a small
latency, but current implementation doesn't exactly meet the
expectations.

When a timer fires but finds the socket is owned by the user, it rearms
itself for an additional delay hoping next run will be more
successful.

tcp_write_timer() for example uses a 50ms delay for next try, and it
defeats many attempts to get predictable TCP behavior in term of
latencies.

Use the recently introduced tcp_release_cb(), so that the user owning
the socket will call various handlers right before socket release.

This will permit us to post a followup patch to address the
tcp_tso_should_defer() syndrome (some deferred packets have to wait
RTO timer to be transmitted, while cwnd should allow us to send them
sooner)

Signed-off-by: Eric Dumazet <[email protected]>
Cc: Tom Herbert <[email protected]>
Cc: Yuchung Cheng <[email protected]>
Cc: Neal Cardwell <[email protected]>
Cc: Nandita Dukkipati <[email protected]>
Cc: H.K. Jerry Chu <[email protected]>
Cc: John Heffner <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

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