blkparse: Fix blktrace output pipe broken in the new kernel

blkparse: Fix blktrace output pipe broken in the new kernel

With the newest kernel(say 2.6.37, some older one should also have the
similar problem), some cfq messages are added to blktrace, so it makes
the old blkparse broken.

See a simple example:
1. blktrace /dev/sdb -o -|blkparse -i -
2. Run the following command(/dev/sdb1 is mounted at /mnt/test_dir):
dd if=/mnt/test_dir/test of=/dev/null bs=4k count=1 iflag=direct

There are only 2 lines of output there:
  8,16   0        1     0.000000000 13183  A   R 114759 + 8 <- (8,17) 114696
  8,16   0        2     0.000000491 13183  Q   R 114759 + 8 [dd]

And even we run a command line like:
for((i=0;i<100;i++))do dd if=/mnt/ocfs2/test of=/dev/null bs=4k count=1 iflag=direct;done
We are only given the same 2 lines of output.

While the really one should look like:
  8,16   0        1     0.000000000 13319  A   R 114759 + 8 <- (8,17) 114696
  8,16   0        2     0.000000376 13319  Q   R 114759 + 8 [dd]
  8,16   0        0     0.000005931     0  m   N cfq13319 alloced
  8,16   0        3     0.000006259 13319  G   R 114759 + 8 [dd]
  8,16   0        4     0.000007143 13319  P   N [dd]
  8,16   0        5     0.000007817 13319  I   R 114759 + 8 [dd]
  8,16   0        0     0.000008491     0  m   N cfq13319 insert_request
  8,16   0        0     0.000009029     0  m   N cfq13319 add_to_rr
...

The main reason is that in show_entries_rb, we test sequences every time,
but actually with some messages like cfq, the sequence number is always
0 which makes the old sequence check refuses all the logs after it.
So only check/store sequence number if it isn't a message.

Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>