blktrace: don't let the sysfs interface remove trace from running list

Currently, blktrace can be started/stopped via its ioctl-based interface
(used by the userspace blktrace tool) or via its ftrace interface. The
function blk_trace_remove_queue(), called each time an "enable" tunable
of the ftrace interface transitions to zero, removes the trace from the
running list, even if no function from the sysfs interface adds it to
such a list. This leads to a null pointer dereference.  This commit
changes the blk_trace_remove_queue() function so that it does not remove
the blk_trace from the running list.

v2:
    - Now the patch removes the invocation of list_del() instead of
      adding an useless if branch, as suggested by Namhyung Kim.

Signed-off-by: Arianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>