[BLOCK] ll_rw_blk: Enable out-of-order request completions through softirq

Request completion can be a quite heavy process, since it needs to
iterate through the entire request and complete the bio's it holds.
This patch adds blk_complete_request() which moves this processing
into a dedicated block softirq.

Signed-off-by: Jens Axboe <axboe@suse.de>

include/linux/interrupt.h

@@ -112,6 +112,7 @@ enum
 	TIMER_SOFTIRQ,
 	NET_TX_SOFTIRQ,
 	NET_RX_SOFTIRQ,
+	BLOCK_SOFTIRQ,
 	SCSI_SOFTIRQ,
 	TASKLET_SOFTIRQ
 };