Revert "tcp_output(): maximize LSO frames size"

This reverts commit 902535a0a88e73f5373843da07d6452266bf8f24.

Raphael reports the following breakage in tst-poll.so:

  OSv
  ---

    TEST tst-nway-merger.so        OK  (0.782 s)
    TEST tst-openat.so             OK  (0.731 s)
    TEST tst-pipe.so               OK  (6.840 s)
    TEST tst-poll.so              OSv v0.10-319-g1bdd4a2
  eth0: 192.168.122.15
  Running 3 test cases...
  Assertion failed: tp->t_flags & 0x1000000
  (/data/osv/bsd/sys/netinet/tcp_timer.cc: tcp_timer_tso_flush: 438)

  [backtrace]
  0x0000000000223e98 <__assert_fail+24>
  0x0000000000279de4 <???+2596324>
  0x00000000003dbd9d <async::timer_task::fire(async::percpu_timer_task&)+125>
  0x00000000003dcb13 <async::async_worker::run()+1587>
  0x00000000003be11a <thread_main_c+26>
  0x0000000000372525 <???+3613989>

  GDB
  ---

  (gdb) bt
  #0  0x00000000003d2a72 in cli_hlt () at /data/osv/arch/x64/processor.hh:242
  #1  halt_no_interrupts () at /data/osv/arch/x64/arch.hh:48
  #2  osv::halt () at /data/osv/core/power.cc:34
  #3  0x0000000000223df0 in abort (fmt=fmt@entry=0x5c9980 "Assertion
  failed: %s (%s: %s: %d)\n") at /data/osv/runtime.cc:139
  #4  0x0000000000223e99 in __assert_fail (expr=<optimized out>,
  file=<optimized out>, line=<optimized out>, func=<optimized out>)
      at /data/osv/runtime.cc:145
  #5  0x0000000000279de5 in tcp_timer_tso_flush (timer=...,
  tp=0xffffa0003db51800) at /data/osv/bsd/sys/netinet/tcp_timer.cc:438
  #6  0x00000000003dbd9e in async::timer_task::fire
  (this=this@entry=0xffffa0003d6b9a10, task=...) at
  /data/osv/core/async.cc:362
  #7  0x00000000003dcb14 in fire (task=..., this=0xffff80003fa84030) at
  /data/osv/core/async.cc:227
  #8  async::async_worker::run (this=0xffff80003fa84030) at
  /data/osv/core/async.cc:175
  #9  0x00000000003be11b in main (this=0xffff80003fa84730) at
  /data/osv/core/sched.cc:915
  #10 sched::thread_main_c (t=0xffff80003fa84730) at
  /data/osv/arch/x64/arch-switch.hh:137
  #11 0x0000000000372526 in thread_main () at /data/osv/arch/x64/entry.S:113
  (gdb) print tp->t_flags
  No symbol "tp" in current context.
  (gdb) frame 5
  #5  0x0000000000279de5 in tcp_timer_tso_flush (timer=...,
  tp=0xffffa0003db51800) at /data/osv/bsd/sys/netinet/tcp_timer.cc:438
  438     KASSERT(tp->t_flags & TF_TSO, "tcp_timer_tso_flush: TSO disabled");
  (gdb) print tp->t_flags
  $1 = 992

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>