tcp: tsq: restore minimal amount of queueing

After commit c9eeec26e32e ("tcp: TSQ can use a dynamic limit"), several
users reported throughput regressions, notably on mvneta and wifi
adapters.

802.11 AMPDU requires a fair amount of queueing to be effective.

This patch partially reverts the change done in tcp_write_xmit()
so that the minimal amount is sysctl_tcp_limit_output_bytes.

It also remove the use of this sysctl while building skb stored
in write queue, as TSO autosizing does the right thing anyway.

Users with well behaving NICS and correct qdisc (like sch_fq),
can then lower the default sysctl_tcp_limit_output_bytes value from
128KB to 8KB.

The new usage of sysctl_tcp_limit_output_bytes permits each driver
author to check how driver performs when/if the value is set
to a minimum of 4KB :

Normally, line rate for a single TCP flow should be possible,
but some drivers rely on timers to perform TX completion and
too long delays prevent reaching full throughput.

Fixes: c9eeec26e32e ("tcp: TSQ can use a dynamic limit")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Sujith Manoharan <sujith@msujith.org>
Reported-by: Arnaud Ebalard <arno@natisbad.org>
Cc: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>

(Cherry picked from http://patchwork.ozlabs.org/patch/290672/)