block: export block_unplug tracepoint

This allows stacked devices (like md/raid5) to provide blktrace
tracing, including unplug events.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>