blktrace: fix context-info when mixed-using blk tracer and trace events

When current tracer is set to blk tracer, TRACE_ITER_CONTEXT_INFO is
unset, but actually context-info is printed:

    pdflush-431   [000]   821.181576:   8,0    P   N [pdflush]

And then if we enable TRACE_ITER_CONTEXT_INFO:

    # echo context-info > trace_options

We'll see context-info printed twice. What's worse, when we use blk
tracer and trace events at the same time, we'll see no context-info
for trace events at all:

    jbd2_commit_logging: dev dm-0:8 transaction 333227
    jbd2_end_commit: dev dm-0:8 transaction 333227 head 332814
      rm-25433 [001]  9578.307485:   8,18   m   N cfq25433 slice expired t=0
      rm-25433 [001]  9578.307486:   8,18   m   N cfq25433 put_queue

This patch adds blk_tracer->set_flags(), and context-info flag is unset
only when we set the output to classic mode.

Note after this patch, one should unset context-info explicitly if he
wants to get binary output that can be parsed by blkparse:

    # echo nocontext-info > trace_options
    # echo bin > trace_options
    # echo blk > current_tracer
    # cat trace_pipe | blkparse -i -

Reported-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <49E54E60.50408@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>