block: don't kick empty queue in blk_drain_queue()

While probing, fd sets up queue, probes hardware and tears down the
queue if probing fails.  In the process, blk_drain_queue() kicks the
queue which failed to finish initialization and fd is unhappy about
that.

  floppy0: no floppy controllers found
  ------------[ cut here ]------------
  WARNING: at drivers/block/floppy.c:2929 do_fd_request+0xbf/0xd0()
  Hardware name: To Be Filled By O.E.M.
  VFS: do_fd_request called on non-open device
  Modules linked in:
  Pid: 1, comm: swapper Not tainted 3.2.0-rc4-00077-g5983fe2 #2
  Call Trace:
   [<ffffffff81039a6a>] warn_slowpath_common+0x7a/0xb0
   [<ffffffff81039b41>] warn_slowpath_fmt+0x41/0x50
   [<ffffffff813d657f>] do_fd_request+0xbf/0xd0
   [<ffffffff81322b95>] blk_drain_queue+0x65/0x80
   [<ffffffff81322c93>] blk_cleanup_queue+0xe3/0x1a0
   [<ffffffff818a809d>] floppy_init+0xdeb/0xe28
   [<ffffffff818a72b2>] ? daring+0x6b/0x6b
   [<ffffffff810002af>] do_one_initcall+0x3f/0x170
   [<ffffffff81884b34>] kernel_init+0x9d/0x11e
   [<ffffffff810317c2>] ? schedule_tail+0x22/0xa0
   [<ffffffff815dbb14>] kernel_thread_helper+0x4/0x10
   [<ffffffff81884a97>] ? start_kernel+0x2be/0x2be
   [<ffffffff815dbb10>] ? gs_change+0xb/0xb

Avoid it by making blk_drain_queue() kick queue iff dispatch queue has
something on it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Ralf Hildebrandt <Ralf.Hildebrandt@charite.de>
Reported-by: Wu Fengguang <fengguang.wu@intel.com>
Tested-by: Sergei Trofimovich <slyich@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>