[PATCH] fix blk_start_queue()

[PATCH] fix blk_start_queue()

Here's the second attempt at fixing blk_start_queue().  The only change
since last version is using proper atomic bitops.  If we moved the
read/write full to a different variable, we could rely on the queue lock
for plugging and stop/start of queue (by far the most used bit
operations there) and skip the atomic bitops.